Novel sequence variants of multi-drug resistance genes, mdr1 and mrp1, and recombinant cells expressing mrp1 and mdr1 for assessment of drug penetration and disposition

ABSTRACT

Provided are compositions relating to novel MDR1 polymorphisms, including nucleic acids, polypeptides, and recombinant cells, as well as methods for detection of MDR1 polymorphisms in biological samples and elucidation of the influence of MDR1 polymorphisms on MDR1 protein function. Also provided are a rat MRP1 cDNA and protein, stable cell lines expressing the rat MRP1 protein, and methods of assessing drug penetration or disposition in a cell line expressing a recombinant mammalian MRP1 or MDR1 protein, or a homolog thereof.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No. 11/045,578, filed Jan. 26, 2005, which claims the benefit of Provisional Patent Application No. 60/539,362, filed Jan. 26, 2004, and the entire disclosure of which is incorporated by reference herein, priority from the filing date of which is hereby claimed under 35U.S.C. § 120.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

The research was supported by National Institutes of Health grants GM 62883, AI-52663, NS39178, AI31854, ES007033, and HL56548 and by NIH Pharmaceutical Sciences Training Grant GM07750. The Government has certain rights in the invention.

BACKGROUND

Transporter proteins constitute a significant fraction of membrane-bound proteins, which account for approximately 30% of total proteins found in humans. About half of the membrane-bound transporters reported to date have been discovered within the past 5 years. The role of membrane transporters located in various organs and tissues in drug delivery and their disposition in vivo is just beginning to be appreciated. For example, a major challenge in clinical pharmacology is to elucidate the often observed, wide inter-individual variability in pharmacokinetics. Recently, Greiner et al. (J. Clin. Invest. 104:147 (1999)) demonstrated an inter-individual variation in the intestinal expression of the multidrug resistance transporter protein, MDR1 or P-glycoprotein that resulted in a 7-fold range in the oral bioavailability of a P-glycoprotein (pgp) substrate, digoxin, in healthy human subjects. Therefore, it is important to understand the role of transporters in the disposition of currently available drugs, as well as to develop the capability of predicting the contribution of membrane transporters in the disposition of new molecular entities under development.

There are two well-characterized major ATP-binding cassette (ABC) superfamily members, ABC-B and ABC-C, involved in conferring drug resistance to cancer cells (Higgins, Annu. Rev. Cell. Biol. 8:67-113 (1992); Wijnholds, Novartis Found. Symp. 243:69-79, discussion 80-2, 180-5 (2002)). Within these two families, two protein isoforms, one from each family, play a critical role in drug resistance. They are P-glycoprotein (Pgp or MDR1), which belongs to the ABC-B family (Gottesman and Pastan, Annu. Rev. Biochem. 62:385-427 (1993)), and MRP1, which belongs to the ABC-C family (Cole et al., Cancer Res. 54(22):5902-10 (1994)). Pgp is the most extensively studied ATP-binding cassette transporter, functioning as a biological barrier by extruding toxic substances and xenobiotics out of the cells. In the clinical setting, the over-expression of multidrug resistance proteins such as MDR1 or MRP1 is often associated with poor prognosis for cancer patients.

Progress has been made in understanding of the structure-function relationship of some of the major drug transporters in recent years, notably the ABC transporters, and ABCB1 or MDR1 in particular. While certain amino acid residues in the drug and co-substrate (nucleotide) binding domains and those involved in the anchoring of the membrane-spanning polypeptides have been identified through site-directed mutagenesis and single nucleotide polymorphism studies, the actual biophysical mechanism of solute translocation is far from being elucidated.

The human MDR1 gene encodes a 170 kilodalton integral membrane protein that mediates ATP-dependent substrate efflux. The protein product, P-glycoprotein, a member of the ATP-binding cassette (ABC) superfamily of transporters, resides in the plasma membrane and functions as an efflux transporter of a wide variety of natural compounds and lipophilic xenobiotics. While the contribution of P-glycoprotein in multidrug resistance for cancer chemotherapy is well documented, the role of P-glycoprotein in drug disposition is not fully understood and has continued to generate significant debate. P-glycoprotein mediates the energy-dependent efflux of a broad range of xenobiotics in epithelial tissues throughout the human body including the intestinal mucosa, liver canalicular membrane, kidney proximal tubules, blood-brain-barrier, and placenta (Schinkel, Semin. Cancer Biol. 8:161-70 (1997)). Because P-glycoprotein is found in tissues important in drug disposition, variation in expression and function of P-glycoprotein due to genetic polymorphisms of MDR1 may influence pharmacokinetics and, in turn, pharmacodynamics.

Despite some similarities in drug resistance profiles, the MDR1 and MRP1 transporters differ somewhat in substrate selectivity, molecular structure, tissue distribution, and membrane location in cells. At the genetic level, MDR1 and MRP1 are only 15% identical in their amino acid sequences (Kruh et al., J. Bioenerg. Biomembr. 33(6):493-501 (2001)). MRP1 is capable of transporting many lipophilic anions and conjugated substances. The MRP1 substrate includes a variety of structurally diverse anticancer drugs, GSH-conjugates, glucuronides, leukotriene C4 (LTC4), unmodified drugs, and some drugs that are multivalent organic anions, while Pgp substrates are mostly natural or mildly cationic molecules. A few previous studies (Paul et al., Biochem. 35(42):13647-55 (1996); Stride et al., Mol. Pharmacol. 52(3):344-53 (1997); Zhang et al. J. Biol. Chem. 276(16):13231-9 (2001)) suggest that there are differences in substrate recognition and transport activity between the mouse and human ortholog of MRP1, indicating that significant interspecies differences exist for MRP1.

In rat, only a few reports are available for MRPs. It has been shown that the rat MRP1 expression occurs in almost all tissues, and its over-expression could confer multidrug resistance in cancer cells (Chemington et al., J. Pharmacol. Exp. Ther. 300(1):97-104 (2002)). Expression of rat MRP1 in the BBB capillary system and placenta has also been reported (Regina et al., J. Neurochem. 71 (2):705-15 (1998)), suggesting that MRP1 plays certain roles in drug distribution into the central nervous system and in drug exchanges between the mother and fetus.

There are significant challenges in elucidating the functioning of transporters at the cellular and organ levels. Several reviews (Kunta and Sinko, Curr. Drug Metab. 5:109-124 (2004); van Montfoort et al., Curr. Drug Metab. 4:185-211 (2003)) have emphasized the concerted action of apical and basolateral membrane transporters in the translocation of a solute across epithelial barriers. The directionality of transport often requires carefully controlled studies with a cell barrier model. While cell barrier models for the intestinal and renal tubular epithelia are well-validated (e.g., Caco-2 LLC-PK1 and MDCK) and generally adopted, such models are still not available for the blood-brain barrier, the placenta, and the nasal and bronchial epithelia for direct evaluation of individual transporter proteins of interest such as MDR1 product Pgp and its genetic variants.

Research advances on the pharmacogenetics of human drug-metabolizing enzymes over the past two decades have afforded valuable insights into the clinical pharmacokinetics of a wide range of drugs, which have in turn led to clearer understanding of the metabolic basis of drug interactions and individual susceptibility to drug toxicities. It has had a profound impact on drug development and regulation.

The same impact may be observed as progress is made toward better understanding of the functional impact of genetic variations with polymorphically-expressed drug transporters. The genetic complexity of transporter genes is just beginning to be understood (see, e.g., Woodahl and Ho, Curr. Drug Metab. 5:11-19 (2004)). Linking the genetic polymorphism to a clinically discernable phenotype is complicated by linkage disequilibrium among single nucleotide polymorphisms. Improvement in molecular genetic technologies that could validate computational approaches that are currently used to assign and predict haplotypes could make a significant contribution in this area. Beyond the constitutive gene expression are issues related to gene regulation, such as induction and suppression of transporter gene transcription by endo- and xenobiotics, tissue-specific regulation through alternate splicing and post-translational modification, and modulation by diseases or pathophysiology.

Improved knowledge of drug transporters will optimize the delivery of a drug molecule to the target site(s) of interest. Discovery of new genetic variants of MDR1 that may have significant impact on drug penetration to cell and tissues as well as the availability of validated in vitro tools to predict drug availability and potential interactions with drugs that are given concomitantly could provide a significant improvement accelerating drug development and improve drug safety.

SUMMARY

In one aspect, the present invention provides a method for predicting responsiveness to a drug in a subject, where subject has a disease or disorder amenable to treatment with the drug and the drug is a substrate of MDR1. The method generally includes the following steps: (1) isolating a biological sample that includes nucleic acids and/or proteins from the subject; and (2) analyzing the biological sample to determine (a) the nucleotide at position 1199 of the MDR1 gene and/or (b) the presence or absence of the S4001 variant of MDR1 protein; whereby the presence of T at nucleotide position 1199 or the presence of the S4001 MDR1 variant is indicative of an increased responsiveness to the drug in the subject relative an individual that is homozygous for a wild-type MDR1 gene. In typical embodiments, the biological sample includes nucleic acids and the analyzing step comprises determining the nucleotide present at position 1199 of the MDR1 gene. A particularly suitable technique for analyzing the nucleic acids in the sample includes hybridization between the nucleic acid sample and a nucleic acid that is either (a) a nucleic acid including about 10 to about 100 contiguous nucleotides of a G1199T variant of the nucleotide sequence set forth in SEQ ID NO:5, where the nucleic acid includes the nucleotide at position 1199 and/or a base adjacent thereto or (b) a nucleic acid that is fully complementary-to the nucleic acid of (a). In certain variations, the drug is a cytotoxic chemotherapeutic agent such as, e.g., doxorubicin, paclitaxol, vinblastine, and vincristine.

In another aspect, the present invention provides a method for predicting oral bioavailability of an HIV protease inhibitor in a subject, where the HIV protease inhibitor is a substrate of MDR1. The method generally includes the following steps: comprising: (1) isolating a biological sample that includes nucleic acids and/or proteins from the subject; and (2) analyzing the biological sample to determine (a) the nucleotide at position 1199 of the MDR1 gene, and/or (b) the presence or absence of the S400N variant of MDR1 protein, whereby the presence of A at nucleotide position 1199 or the presence of the S400N variant is indicative of a decreased oral bioavailability of the HIV protease inhibitor in the subject relative an individual that is homozygous for a wild-type MDR1 gene. In typical embodiments, the biological sample includes nucleic acids and the analyzing step comprises determining the nucleotide present at position 1199 of the MDR1 gene. A particularly suitable technique for analyzing the nucleic acids in the sample includes hybridization between the nucleic acid sample and a nucleic acid that is either (a) a nucleic acid including about 10 to about 100 contiguous nucleotides of a G1199A variant of the nucleotide sequence set forth in SEQ ID NO:5, where the nucleic acid includes the nucleotide at position 1199 and/or a base adjacent thereto, or (b) a nucleic acid that is fully complementary to the nucleic acid of (a). Exemplary HIV protease inhibitors amenable to prediction of oral biovailability in accordance with the above method include, e.g., amprenavir, indinavir, lopinavir, ritonavir, and saquinavir. In certain variations, the method further includes selecting a mode of administration for the HIV protease inhibitor based on whether a decreased oral bioavailability is indicated.

In certain embodiments of the present invention, the above methods further include determining the MDR1 genotype at nucleotide position 1199 of the MDR1 gene.

In yet another aspect, the present invention provides an isolated nucleic acid that is (a) a nucleic acid that includes at least 15 contiguous nucleotides of a variant of the nucleotide sequence set forth in SEQ ID NO:5, where (i) the variant is A163C and the nucleic acid includes the nucleotide at position 163; (ii) the variant is A886C and the nucleic acid includes the nucleotide at position 886; (iii) the variant is G1199T and the nucleic acid includes the nucleotide at position 1199; (iv) the variant is G1292T/T1293G and the nucleic acid includes at least one of the nucleotide at position 1292 and the nucleotide at position 1293; (v) the variant is T2814G and the nucleic acid includes the nucleotide at position 2814; (vi) the variant is C3258T and the nucleic acid includes the nucleotide at position 3258; or (vii) the variant is G3271C and the nucleic acid includes the nucleotide at position 3271; or (b) a nucleic acid that is fully complementary to the nucleic acid of (a). In certain embodiments, the nucleic acid encodes the corresponding variant MDR1 protein or a fragment thereof having MDR1 activity. The present invention also provides an expression vector that includes any one of the above nucleic acids.

In still another aspect, the present invention provides an isolated MDR1 protein that is a variant of the wild-type MDR1 protein having the amino acid sequence set forth in SEQ ID NO:4, in which the MDR1 variant has at least one of the following amino acid substitutions: T55P, N296H, S4001, C431L, F938L, and A1091P. In one specific embodiment, the isolated MDR1 has the S400I amino acid substitution.

Also provided is an isolated cell line expressing an MDR1 protein as set forth above. In certain embodiments, the isolated cell line is a stable recombinant cell line (such as, e.g., a stably transformed epithelial cell line) that comprises the expression vector that includes a nucleic acid as set forth above. In specific variation, the cell line expresses an MDR1 variant having the S400I amino acid substitution.

In another aspect, the present invention provides a method for evaluating the influence of a MDR1 polymorphism on MDR1 mediated efflux, uptake, or transcellular transport, the method including the following steps: (1) contacting a first cell with a therapeutic agent, where the first cell expresses an MDR1 variant having at least one amino acid substitution selected from T55P, N296H, S4001, C431L, F938L, and A1091P; (2) contacting a control cell with the therapeutic agent, where the control cell expresses the wild type MDR1 protein; (3) culturing the first cell and control cell; (4) determining for each of the first cell and control cell at least one of the uptake, efflux, or transcellular transport of the therapeutic agent; and (5) comparing the level of uptake, efflux, or transcellular transport of the therapeutic agent in the first cell with that of the control cell.

In yet another aspect, the present invention provides a stably transformed cell line capable of transcellular transport and comprising a non-viral vector encoding and expressing a recombinant mammalian MRP1 or MDR1 protein, or a homolog thereof. In certain embodiments, the cell line is an epithelial cell line such as, e.g., a renal or intestinal epithelial cell line. In specific variations, the cell line is LLC-PK1, MDCKII, or Caco-2.

Still further, the present invention provides methods for using the foregoing cell line. For example, in one embodiment, a method for evaluating the contribution of an MRP1 or MDR1 protein to the disposition or penetration of an agent is provided that includes contacting the cell line (capable of transcellular transport and comprising a non-viral vector encoding and expressing a recombinant mammalian MRP1 or MDR1 protein, or a homolog thereof) with the agent; culturing the contacted cell line; and determining at least one of the uptake, efflux, or transcellular transport of the agent. In another embodiment, a method for evaluating the influence of a MDR1 or MRP1 polymorphism on MDR1- or MRP1-mediated efflux, uptake, or transcellular transport, is provided that includes the following steps: (1) contacting a first cell (capable of transcellular transport and comprising a non-viral vector encoding and expressing a recombinant mammalian MRP1 or MDR1 protein, or a homolog thereof) with a therapeutic agent, where the recombinant MRP1 or MDR1 protein expressed in the first cell is an allelic variant of a wild-type MRP1 or MDR1 protein; (2) contacting a control cell with the therapeutic agent, where the control cell expresses the wild type MRP1 or MDR1 protein corresponding to the allelic variant; (3) culturing the first cell and control cell; (4) determining for each of the first cell and control cell at least one of the uptake, efflux, or transcellular transport of the therapeutic agent; and (5) comparing the level of uptake, efflux, and/or transcellular transport of the therapeutic agent in the first cell with that of the control cell.

DEFINITIONS

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar to those described herein can be used in the practice or testing of the present invention, only exemplary methods and materials are described. For purposes of the present invention, the following terms are defined below.

The terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

An “agent” refers to any molecule with a potential to structurally interact with biomolecules, particularly proteins, through non-covalent interactions, such as, for example, through hydrogen bonds, ionic bonds, van der Waals attractions, or hydrophobic interactions. Agents include, for example, small molecules, nucleic acids, peptides, peptidomimetics, synthetic compounds, and/or natural compounds. Agents include drugs or pharmaceutical agents. A “drug,” “pharmaceutical agent,” or “therapeutic agent” means any substance used in the prevention, diagnosis, alleviation, treatment or cure of a disease, or that is in development for such use, including drugs undergoing testing in animal studies or clinical trials in humans.

“Amenable to treatment” with a drug means that a disorder or disease is either predicted or determined to be a disorder or disease that can be treated by administration of the drug (for example, through clinical testing such as by, e.g., clinical trials conducted to obtain governmental approval of a drug).

As used herein, the term “nucleic acid” refers to a deoxyribonucleotide or ribonucleotide polymer in either single- or double-stranded form and, unless otherwise limited, encompasses known analogues of natural nucleotides that hybridize to nucleic acids in manner similar to naturally occurring nucleotides. The term “nucleic acid” encompasses the terms “oligonucleotide” and “polynucleotide.” The term “oligonucleotide” refers to at least one molecule of between about 3 and about 100 bases in length. The term “polynucleotide” refers to at least one molecule of greater than about 100 bases in length. Unless otherwise indicated, a particular nucleic acid sequence includes the complementary sequence thereof.

As used herein, the term “promoter” refers to a control sequence that controls the initiation and rate of transcription. A promoter can contain genetic elements at which regulatory proteins and molecules bind, such as RNA polymerase and other transcription factors, to initiate the specific transcription of a nucleic acid sequence. A promoter can also include an enhancer. The phrases “operatively positioned,” “operatively linked,” and “operatively associated” mean that a promoter is in a correct functional location and/or orientation in relation to a nucleic acid sequence to control transcriptional initiation and/or expression of a downstream sequence.

As used herein, “wild-type gene” refers to a sequence of nucleic acid, at a genetic locus in the genome of an organism, which encodes a gene product that performs the normal function of the gene product encoded by a naturally occurring nucleotide sequence corresponding to the genetic locus. A genetic locus can have more than one sequence or allele in a population of individuals, and the term “wild-type” encompasses all such naturally-occurring alleles that encode a gene product performing the normal function. “Wild-type” also encompasses gene sequences that are not necessarily naturally occurring, but that still encode a gene product with normal function (e.g., genes having silent mutations or encoding proteins with conservative substitutions).

The term “wild-type” with reference to an MDR1 polypeptide or peptide means an MDR1 polypeptide or peptide encoded by a wild-type MDR1 gene. A genetic locus can have more than one sequence or allele in a population of individuals, and the term “wild-type” encompasses all such naturally-occurring alleles that encode a gene product performing the normal function. In addition to naturally-occurring amino acid sequence variants having normal function, wild-type MDR1 polypeptides include derivatives altered by substitution, addition or deletion of one or more amino acid residues that provide for molecules having normal MDR1 function.

As used herein, the terms “polypeptide,” “peptide” and “protein” are used interchangeably herein to refer to polymer of amino acid residues of varying lengths. A “fragment” of a protein typically comprises or consists of at least 8-10 contiguous amino acids. In other embodiments, the fragment comprises at least 20 or at least 50 contiguous amino acids. In other embodiments, the fragments are not larger than 35, 100, or 200 amino acids. A derivative is a polypeptide having conservative amino acid substitutions, as compared with another sequence. Derivatives further include, for example, glycosylations, acetylations, phosphorylations, and the like. Further included within the definition of “polypeptide” are, for example, polypeptides containing one or more analogs of an amino acid (e.g., unnatural or “non-classical” amino acids, and the like), polypeptides with substituted linkages as well as other modifications known in the art, both naturally and non-naturally occurring.

As used herein, the term “polymorphic” means that multiple variants exists (i.e., two or more alleles exist) at a genetic locus in the individuals of a population. A “polymorphism” refers to a mutation present in 1% or more of alleles of the general population.

As used herein, a “homolog” of a first gene refers to a second, different gene that is substantially identical to the first gene, or that encodes a gene product that is substantially identical to the gene product encoded by the first gene. An “ortholog” of a first gene refers to a second gene from a different organism that is substantially identical to the first gene, or that encodes a gene product that is substantially identical to the gene product encoded by the first gene.

The term “isolated” refers to nucleic acid, protein, polypeptide or fragment thereof that has been removed from its natural cellular environment.

The term “cell line,” as used herein, refers to individual cells, harvested cells, and cultures containing the cells (e.g., in vitro), so long as they are derived from cells of the cell line referred to. A cell line is said to be “continuous,” “immortal,” or “stable” if the line remains viable over a prolonged time, typically at least about six months. To be considered a cell line, as used herein, the cells typically remain viable for at least 50 passages. A “primary cell line,” or “normal cell line,” in contrast, refers to cells that do not remain viable over a prolonged time in culture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Novel cDNA sequence encoding rat MRP1. Rat (SEQ ID NO: 1), mouse (SEQ ID NO:2), and human (SEQ ID NO:3) MRP1 cDNA sequence alignment.

FIG. 2: Typical MDR1 or MRP1 cDNA plasmids designed to express in mammalian cells for generating stable recombinant cell-lines. Plasmid described in Panel A contains poly histidine tail to improve efficiency in isolation of recombinant protein, while the plasmid in Panel B lacks poly histidine tail. Both plasmids (Panel A and B) contain T7 promoter sequences to synthesize RNA standards for measurement of MDR1 (and MRP1) transcript levels in clinical samples. The same set of MDR1 and MRP1 plasmid vehicles could be used as standards for genotype determination.

FIG. 3: Schematic representation of the generation of the MDR1₁₁₉₉ plasmid. The MDR1₁₁₉₉ plasmid was generated by introducing a 1199A variant into the original MDR1_(wt) plasmid. The first step in generating the MDR1₁₁₉₉ plasmid utilized the HindIII and 1199A primer sets to generate two fragments each containing 1199A at one end and a HindIII site at the other from the original MDR1_(wt) plasmid. Next, the fragments were annealed and amplified using the HindIII primer set. Finally, the fragment containing 1199A and the pCR3.1-hMDR1 vector were digested with HindIII and relegated. The MDR1₁₁₉₉ plasmid was validated by restriction enzyme and sequencing analyses. MDR1_(wt) and MDR1₁₁₉₉ plasmids contain T7 and CMV promoters and ampicillin- and neomycin-resistance genes.

FIG. 4: Sequence verification of LLC-PK1 cells expressing MDR1_(wt) and MDR1₁₁₉₉. Recombinant MDR1-expressing LLC-PK1 cells were sequenced at nucleotide position 1199; wild-type allele G and variant allele A. Electropherograms of MDR1_(wt) (panel A) and MDR1₁₁₉₉ (panel B) are shown.

FIG. 5: Immunoblot analysis of LLC-PK1 control and recombinant MDR1 cells. Lane 1: P-glycoprotein positive control isolated from P-glycoprotein over-expressed cell line, MES-SA-DX5; lane 2: LLC-PK1 control cells; lane 3: recombinant MDR1_(wt) cells; lane 4: recombinant MDR1₁₁₉₉ cells.

FIG. 6: Confirmation of apical membrane expression of P-glycoprotein by deconvolution immunofluorescent microscopy in recombinant MDR1_(wt) and MDR1₁₁₉₉ cells. LLC-PK1 control cells (panel A), MDR1_(wt) cells (panel B), and MDR1₁₁₉₉ cells (panel C) plated on glass slides. The cells are orientated such that the apical membrane is on top and the basolateral membrane is on bottom.

FIG. 7: P-glycoprotein mediated efflux of R123 in LLC-PK1 control and MDR1 recombinant cells. The intensity of intracellular R123 was evaluated by flow cytometry in LLC-PK1 control and MDR1 cells (panel A): control (dashed line), MDR1_(wt) cells (bold line), and MDR1₁₁₉₉ cells (solid line). The specificity of R123 efflux was evaluated in the absence (dashed line) and presence (solid line) of 1 μM GF120918; LLC-PK1 control cells (panel B); recombinant MDR1_(wt) cells (panel C); recombinant MDR1₁₁₉₉ cells (panel D).

FIG. 8: Transepithelial transport across LLC-PK1 epithelial cells. Transepithelial permeability was evaluated as the amount of R123 transported across an epithelial monolayer from the donor to recipient compartment over a 4-hour time period; LLC-PK1 control cells (panel A); recombinant MDR1_(wt) cells (panel B); recombinant MDR1₁₁₉₉ cells (panel C). R123 transport in the basolateral-to-apical direction without inhibitor (•) and with 1 μM GF120918 (◯) and in the apical-to-basolateral direction without inhibitor (▴) and with 1 μM GF120918 (Δ).

FIG. 9: Immunoblot analysis of MDR1 product, Pgp expression in recombinant HEK cells.

FIG. 10: Cell surface expression of Pgp in recombinant HEK cells expressing MDR1 and G1199 variants. Recombinant HEK cells expressing MDR1_(wt) (Panel A), MDR1_(G1199A) (Panel B), and MDR1_(G1199T) (Panel C) or control plasmid were fluorescently stained with anti-MDR1 antibody (F4) as described in the Materials and Methods. The distribution of antibody positive cells with high fluorescence was analyzed by flow cytometry. Each panel was presented with cell-distribution profile for control cells (dotted lines) as a reference.

FIG. 11: Percent growth when incubated with varying concentrations of Doxorubicin (A) or Ivermectin (B) for 72 hours. FIG. 3: Cells were seeded at 1E4 cells per well on Day-1 in a 96 well plate. Doxorubicin or Ivermectin were added at varying concentrations on Day 0. After 72 hours, the MTS assay was performed and values were % growth compared to no drug control. (HEK •; WT ◯; G1199A ▴; G1199T ▪).

FIG. 12: Apical-to-basolateral efflux transport of HIV protease inhibitors in the absence of GF120918. Transepithelial permeability was evaluated as the percent transport of protease inhibitors across an epithelial monolayer; LLC-PK1 control cells (panel A); MDR1_(wt) cells (panel B); and MDR1₁₁₉₉ cells (panel C). Transport of amprenavir (▴), indinavir (•), lopinavir (Δ), ritonavir (◯), and saquinavir (▪).

FIG. 13: Basolateral-to-apical efflux transport of HIV protease inhibitors in the absence of GF120918. Transepithelial permeability was evaluated as the percent transport of protease inhibitors across an epithelial monolayer; LLC-PK1 control cells (panel A); MDR1_(wt) cells (panel B); and MDR1₁₁₉₉ cells (panel C). Transport of amprenavir (▴), indinavir (•), lopinavir (Δ), ritonavir (◯), and saquinavir (▪).

FIG. 14: Apical-to-basolateral efflux transport of HIV protease inhibitors in the presence of GF120918. Transepithelial permeability was evaluated as the percent transport of protease inhibitors across an epithelial monolayer; LLC-PK1 control cells (panel A); MDR1_(wt) cells (panel B); and MDR1₁₁₉₉ cells (panel C). Transport of amprenavir (▴), indinavir (•), lopinavir (Δ), ritonavir (◯), and saquinavir (▪).

FIG. 15: Basolateral-to-apical efflux transport of HIV protease inhibitors in the presence of GF120918. Transepithelial permeability was evaluated as the percent transport of protease inhibitors across an epithelial monolayer; LLC-PK1 control cells (panel A); MDR1_(wt) cells (panel B); and MDR1₁₁₉₉ cells (panel C). Transport of amprenavir (▴), indinavir (•), lopinavir (Δ), ritonavir (◯), and saquinavir (▪).

FIG. 16: (A) Human MDR1 wild-type protein amino acid sequence (SEQ ID NO:4). (B) Human MDR1 wild-type gene nucleotide sequence (cDNA) (SEQ ID NO:5).

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention relates to a collection of MDR1 transcript sequence variations (or referred to as alleles or polymorphisms, as the context may indicate). Analysis of MDR1 transcripts in leukemia patients has led to discovery of MDR1 genetic variants that influence Pgp (MDR1 protein) functions. Without intending to be bound by any particular theory, these genetic variants may alter effectiveness of drug therapies by at least two well-described mechanisms. (1) Locally, expression of Pgp at cell surface could impact the ability of drug substrates to remain in the therapeutic target cells due to a genetic variation that impacts protein function, and (2) globally, expression of Pgp and variants in epithelial cells found in tissues controlling drug penetration and disposition such as gut, liver, kidney and blood-brain barrier can reduce overall drug availability in target tissues.

The MDR1 variants described can be used to identify individuals carrying MDR1 variants by standard genotyping methods to prevent adverse drug reactions and/or ineffective therapeutic effects due to drug interactions as well as altered drug dispositions. An individual having, or suspected of having, an MDR1 allele can be screened by genotyping methods to confirm or determine the presence of the MDR1 allele or polymorphism. Detection of polymorphisms in the MDR1 gene is useful in designing and performing diagnostic assays for evaluation of an individual's susceptibility to MDR1-mediated efflux, uptake, and/or transport of a drug. Such assessment is useful, for example, in the design of patient-specific treatment regimens, including selection of drug dosing and/or routes of administration.

In other aspects of the present invention, these and other genetic variants can be expressed in cell lines (e.g., stable recombinant epithelial cells) to allow systematic in vitro evaluation of drug candidates with respect to their ability to successfully penetrate the absorption barriers. In parallel, with a similar technology, continuous cell lines that express multidrug resistance associated protein-1 (MRP1) transporter protein are provided. The availability of recombinant continuous cell-lines that express functional MDR1 or MRP1 protein, and its genetic variants, permit detailed evaluation of drug candidates in cell culture to predict potential drug interactions and variations in drug efficacy. These recombinant cells expressing Pgp and/or MRP1 drug efflux transporter will accelerate pre-clinical assessment of the role of this drug efflux transporter in modulating drug levels in tissues such as brain, and in therapeutic target cells.

Detection of MDR1Polymorphisms

The present invention includes nucleic acid molecules that are oligonucleotides capable of hybridizing, under stringent hybridization conditions, with complementary regions of a the MDR1 gene containing a polymorphism of the present invention. A nucleic acid can be DNA or RNA, and single- or double-stranded. Oligonucleotides can be naturally occurring or synthetic, but are typically prepared by synthetic means. Preferred oligonucleotides of the invention include segments of DNA, or their complements, that are polymorphic variants of the wild-type MDR1 sequence shown in SEQ ID NO:5 having any one of the polymorphic sites shown in Table 1. The segments are usually between 5 and 100 contiguous bases, and often range from 5, 10, 12, 15, 20, or 25 nucleotides to 10, 15, 30, 25, 20, 50 or 100 nucleotides. Nucleic acids between 5-10, 5-20, 10-20, 12-30, 15-30, 10-50, 20-50, 10-100, or 20-100 bases are common. The polymorphic site can occur within any position of the segment. Oligonucleotides of the present invention can be RNA, DNA, or derivatives of either. The minimum size of such oligonucleotides is the size required for formation of a stable hybrid between an oligonucleotide and a complementary sequence on a nucleic acid molecule of the present invention. The present invention includes oligonucleotides that can be used as, for example, probes to identify nucleic acid molecules or primers to produce nucleic acid molecules. Also provided are oligonucleotides that can be used as primers to amplify DNA. Preferred oligonucleotide probes or primers include a single base change of a polymorphism of the present invention or the wild-type nucleotide that is located at the same position. Preferably the nucleotide of interest occupies a central position of a probe. Preferably the nucleotide of interest occupies a 3′ position of a primer.

TABLE 1 Genetic variants that encode for human multidrug resistance (MDR1) protein Sequence variation^(a) Functional (Con- Amino acid region of Estimated firmed) variation^(b) interest^(C) Frequency^(a) Comments A→C Thr⁵⁵→Pro⁵⁵ Trans- 0.04 at 163 membrane domain (TM) 1 A→C Asn²⁹⁶→His²⁹⁶ 0.07 At 886 G→A Ser⁴⁰⁰→Asn⁴⁰⁰ Cytoplasmic 0.04 G→A was at 1199 region in described proximity to in nucleotide Cascorbi (NT) et al., Clin binding Pharmaco domain 1 Ther 2001; 69: 169-74 G→T Ser⁴⁰⁰→Ile⁴⁰⁰ 0.02 at 1199 GT→TG Cys⁴³¹→Leu⁴³¹ NT binding 0.04 at 1292-3 domain 1 T→G Phe⁹³⁸→Leu⁹³⁸ TM 12 regions 0.02 at 2814 C→T Phe¹⁰⁸⁷→Phe¹⁰⁸⁷ Hinge regions 0.06 at 3258 (no change in between TM 12 aa) and NT binding domain 2 G→C Ala¹⁰⁹¹→Pro¹⁰⁹¹ Hinge regions 0.04 at 3271 between TM 12 and NT binding domain 2 ^(a)The MDR1 RNA transcripts were reverse-transcribed into complementary DNA and the entire RNA coding for MDR1 product, Pgp protein was analyzed based on wild-type data found in the Genbank data (accession # GI; 187468). Samples from a total to 44 leukemia patients were collected to date and 44 of them have been sequenced for the entire coding regions of MDR1. Estimated frequency of sequence variations in leukemia patients were estimated based on 44 subjects. ^(b)Amino acid sequences were predicted based on the nucleotide sequences. ^(c)Functional region of interest is predicted based on data available in NCBI database (http://www.ncbi.nlm.nih.gov/cgi-in/Entrez/wwwfeat.cgi?gi=2506118&db=p&form=0&to=1000)

Polymorphisms are detected in a target nucleic acid from an individual being analyzed. For assay of genomic DNA, virtually any biological sample (other than pure red blood cells) is suitable. Tissue means any sample taken from any subject, preferably a human. For example, convenient tissue samples include whole blood, semen, saliva, tears, urine, fecal material, sweat, buccal epithelium, skin and hair. For assay of cDNA mRNA, the tissue sample must be obtained from an organ in which the target nucleic acid is expressed.

In particular variations of the methods described herein, a biological sample from a subject is analyzed to determine the MDR1 genotype at one or more polymorphic sites of the MDR1 gene.

Many of the methods described below require amplification of DNA from target samples. This can be accomplished by e.g., PCR. See generally PCR Technology: Principles and Applications for DNA Amplification (ed. H. A. Erlich, Freeman Press, NY, N.Y., 1992); PCR Protocols: A Guide to Methods and Applications (eds. Innis, et al., Academic Press, San Diego, Calif., 1990); Mattila et al., Nucleic Acids Res. 19:4967 (1991); Eckert et al., PCR Methods and Applications 1:17 (1991); PCR (eds. McPherson et al., IRL Press, Oxford); and U.S. Pat. No. 4,683,202, each incorporated by reference herein.

Other suitable amplification methods include the ligase chain reaction (LCR) (see, e.g., Wu and Wallace, Genomics 4:560 (1989), Landegren et al., Science 241:1077 (1988), each incorporated by reference herein), transcription amplification (see, e.g., Kwoh et al., Proc. Natl. Acad. Sci. USA 86:1173 (1989), incorporated by reference herein), self-sustained sequence replication (see, e.g., Guatelli et al., Proc. Nat. Acad. Sci. USA 87:1874 (1990), incorporated by reference herein) and nucleic acid based sequence amplification (NASBA). The latter two amplification methods involve isothermal reactions based on isothermal transcription, which produce both single stranded RNA (ssRNA) and double stranded DNA (dsDNA) as the amplification products in a ratio of about 30 or 100 to 1, respectively.

The identity of bases occupying the polymorphic sites shown in Table 1 can be determined in an individual (e.g., a patient being analyzed) by several methods, which are described as follows:

Single Base Extension Methods

Single base extension methods are described by e.g., U.S. Pat. No. 5,846,710, U.S. Pat. No. 6,004,744, U.S. Pat. No. 5,888,819 and U.S. Pat. No. 5,856,092, each incorporated by reference herein. In brief, the methods work by hybridizing a primer that is complementary to a target sequence such that the 3′ end of the primer is immediately adjacent to, but does not span a site of, potential variation in the target sequence. That is, the primer comprises a subsequence from the complement of a target polynucleotide terminating at the base that is immediately adjacent and 5′ to the polymorphic site. The term primer refers to a single-stranded oligonucleotide capable of acting as a point of initiation of template-directed DNA synthesis under appropriate conditions (i.e., in the presence of four different nucleoside triphosphates and an agent for polymerization, such as DNA or RNA polymerase or reverse transcriptase) in an appropriate buffer and at a suitable temperature. The appropriate length of a primer depends on the intended use of the primer but typically ranges from 15 to 40 nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with a template. The term primer site refers to the area of the target DNA to which a primer hybridizes. The term primer pair means a set of primers including a 5′ upstream primer that hybridizes with the 5′ end of the DNA sequence to be amplified and a 3′, downstream primer that hybridizes with the complement of the 3′ end of the sequence to be amplified. Hybridization probes are capable of binding in a base-specific manner to a complementary strand of nucleic acid.

Such probes include nucleic acids and peptide nucleic acids as described in Nielsen et al., Science 254:1497-1500 (1991), incorporated by reference herein. A probe primer can be labeled, if desired, by incorporating a label detectable by spectroscopic, photochemical, biochemical, immunochemical, or chemical means. For example, useful labels include ³²P, fluorescent dyes, electron dense reagents, enzymes (as commonly used in an ELISA), biotin, or haptens and proteins for which antisera or monoclonal antibodies are available. A label can also be used to “capture” the primer, so as to facilitate the immobilization of either the primer or a primer extension product, such as amplified DNA, on a solid support. The hybridization is performed in the presence of one or more labeled nucleotides complementary to base(s) that may occupy the site of potential variation. For example, for biallelic polymorphisms, two differentially labeled nucleotides can be used. For tetraallelic polymorphisms, four differentially-labeled nucleotides can be used. In some methods, particularly methods employing multiple differentially labeled nucleotides, the nucleotides are dideoxynucleotides. Hybridization is performed under conditions permitting primer extension if a nucleotide complementary to a base occupying the site of variation if the target sequence is present. Extension incorporates a labeled nucleotide thereby generating a labeled extended primer. If multiple differentially-labeled nucleotides are used and the target is heterozygous then multiple differentially-labeled extended primers can be obtained. Extended primers are detected providing an indication of which base(s) occupy the site of variation in the target polynucleotide.

Allele-Specific Probes

The design and use of allele-specific probes for analyzing polymorphisms is described by e.g., Saiki et al., Nature 324, 163-166 (1986); Dattagupta, EP 235,726; and Saiki, WO 89/11548, each incorporated by reference herein. Allele-specific probes can be designed that hybridize to a segment of target DNA from one individual but do not hybridize to the corresponding segment from another individual due to the presence of different polymorphic forms in the respective segments from the two individuals. Hybridization conditions should be sufficiently stringent such that there is a significant difference in hybridization intensity between alleles, and preferably an essentially binary response, whereby a probe hybridizes to only one of the alleles. Hybridizations are usually performed under stringent conditions that allow for specific binding between an oligonucleotide and a target DNA containing one of the polymorphic sites shown in Table 1. Stringent conditions are defined as any suitable buffer concentrations and temperatures that allow specific hybridization of the oligonucleotide to highly homologous sequences spanning at least one of the polymorphic sites shown in Table 1 and any washing conditions that remove non-specific binding of the oligonucleotide. For example, conditions of 5×SSPE (750 mM NaCl, 50 mM Na Phosphate, 5 mM EDTA, pH 7.4) and a temperature of 230 C are suitable for allele-specific probe hybridizations. The washing conditions usually range from room temperature to 60° C. Some probes are designed to hybridize to a segment of target DNA such that the polymorphic site aligns with a central position (e.g., in a 15 mer at the 7 position; in a 16 mer, at either the 8 or 9 position) of the probe. This probe design achieves good discrimination in hybridization between different allelic forms.

Allele-specific probes are often used in pairs, one member of a pair showing a perfect match to a reference form of a target sequence and the other member showing a perfect match to a variant form. Several pairs of probes can then be immobilized on the same support for simultaneous analysis of multiple polymorphisms within the same target sequence. The polymorphisms can also be identified by hybridization to nucleic acid arrays, some examples of which are described by WO 95/11995, incorporated by reference herein.

Allele-Specific Amplification Methods

An allele-specific primer hybridizes to a site on target DNA overlapping a polymorphism and only primes amplification of an allelic form to which the primer exhibits perfect complementarily. See, e.g., Gibbs, Nucleic Acid Res. 17:2427-2448 (1989), incorporated by reference herein. This primer is used in conjunction with a second primer that hybridizes at a distal site. Amplification proceeds from the two primers leading to a detectable product signifying that the particular allelic form is present. A control is usually performed with a second pair of primers, one of which shows a single base mismatch at the polymorphic site and the other of which exhibits perfect complementarily to a distal site. The single-base mismatch prevents amplification and no detectable product is formed. In some methods, the mismatch is included in the 3′-most position of the oligonucleotide aligned with the polymorphism because this position is most destabilizing to elongation from the primer. See, e.g., WO 93/22456, incorporated by reference herein. In other methods, a double-base mismatch is used in which the first mismatch is included in the 3′-most position of the oligonucleotide aligned with the polymorphism and a second mismatch is positioned at the immediately adjacent base (the pen-ultimate 3′ position). This double mismatch further prevents amplification in instances in which there is no match between the 3′ position of the primer and the polymorphism.

Direct-Sequencing

The direct analysis of the sequence of polymorphisms of the present invention can be accomplished using either the dideoxy-chain termination method or the Maxam Gilbert method (see Sambrook et al., Molecular Cloning, A Laboratory Manual (2nd ed., CSHP, New York 1989); Zyskind et al., Recombinant DNA Laboratory Manual, (Acad. Press, 1988), each incorporated by reference herein).

Denaturing Gradient Gel Electrophoresis

Amplification products generated using the polymerase chain reaction can be analyzed by the use of denaturing gradient gel electrophoresis. Different alleles can be identified based on the different sequence-dependent melting properties and electrophoretic migration of DNA in solution. Erlich, ed., PCR Technology, Principles and Applications for DNA Amplification (W. H. Freeman and Co., New York, 1992), Chapter 7, incorporated by reference herein.

Single-Strand Conformation Polymorphism Analysis

Alleles of target sequences can be differentiated using single-strand conformation polymorphism analysis, which identifies base differences by alteration in electrophoretic migration of single stranded PCR products, as described in Orita et al., Proc. Nat. Acad. Sci. USA 86:2766-2770 (1989), incorporated by reference herein. Amplified PCR products can be generated as described above, and heated or otherwise denatured, to form single stranded amplification products.

Single stranded nucleic acids may refold or form secondary structures that are partially dependent upon the base sequence. The different electrophoretic mobilities of single stranded amplification products can be related to base-sequence differences between alleles of target sequences.

Detection of Variant MDR1Proteins

In certain embodiments, a biological sample that includes cellular proteins from a tissue that expresses the MDR1 gene is analyzed for the presence of a polymorphic variant of MDR1. The MDR1 protein in the sample is analyzed for the presence of one or more of the amino acid substitutions resulting from the polymorphism as set forth in Table 1. For example, the determination of the presence in a sample of an MDR1 variant can be carried out as an immunoassay in which the sample is contacted with an antibody capable of binding the MDR1 protein. Antibodies (e.g., monoclonal antibodies) can be raised that specifically distinguish between wild-type MDR1 and an MDR1 variant as set forth in Table 1. Methods for making antibodies are well-known in the art and are described in, e.g., Harlow and Lane, Antibodies: A Laboratory Manual (Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. 1988), incorporated by reference herein.

Methods of Using MDR1Polymorphisms

After determining polymorphic form(s) present in an individual at one or more polymorphic sites, this information can be used in a number of methods.

For example, detection of MDR1 polymorphisms is useful for conducting clinical trials of drug candidates. Such trials may be performed on treated or control populations having similar or identical polymorphic profiles at a defined collection of polymorphic sites. Use of genetically matched populations eliminates or reduces variation in treatment outcome due to genetic factors, leading to a more accurate assessment of the efficacy of a potential drug.

Furthermore, the detection of MDR1 polymorphisms may be used after the completion of a clinical trial to elucidate differences in response to a given treatment. For example, the methods described herein may be used to identify subsets of patients with similar polymorphic profiles who have unusual (high or low) response to treatment or who do not respond at all (non-responders). In this way, information about the underlying genetic factors influencing response to treatment can be used in many aspects of the development of treatments (these range from the identification of new targets, through the design of new trials to product labeling and patient targeting). Additionally, the methods may be used to identify the genetic factors involved in adverse response to treatment (adverse events). For example, patients who show an adverse response may have a higher incidence of the absence of a particular MDR1 polymorphism than observed in the general population. This would allow the early identification and exclusion of such individuals from treatment. It would also provide information that might be used to understand the biological causes of adverse events and to modify the treatment to avoid such outcomes.

As with other human polymorphisms, the polymorphisms of the invention also have more general applications, such as forensic, paternity testing, linkage analysis and positional cloning.

In particular variations of the present invention, a biological sample from a subject is analyzed to determine the presence or absence of one or more MDR1 polymorphisms (e.g., as set forth in Table 1) to predict responsiveness to the drug in the subject, or to predict one or more aspects of drug disposition or penetration in vivo (e.g., absorption for oral bioavailability, blood-brain barrier penetration, or systemic clearance via secretion and elimination by kidney and liver). For example, as described further hereinbelow (see Example 2), recombinant cells expressing G1199T variant of MDR1 (MDR1_(G1199T)) show an increased sensitivity to drugs that are substrates of MDR1 relative to cells expressing only wild-type MDR1. Subjects having a disease or disorder which is amenable to treatment with a drug that is an MDR1 substrate and expressing the MDR1_(G1199T) allele are predicted to demonstrate increased responsiveness to the drug (increased drug efficacy) relative to an individual that is homozygous for the wild-type MDR1 gene.

Accordingly, in certain variations of the present invention, a method for predicting efficacy a drug in a subject in need thereof, where the drug is a substrate of MDR1, includes (1) isolating a biological sample from the subject, the biological sample comprising nucleic acids and/or proteins; and (2) analyzing the biological sample to determine at least one of (a) the nucleotide at position 1199 of the MDR1 gene, and (b) the presence or absence of the S4001 variant of MDR1 protein, where the presence of T at nucleotide position 1199 or the presence of the S4001 variant is indicative of an increased efficacy of the drug in the subject relative an individual that is homozygous for a wild-type MDR1 gene. A particularly suitable technique for analysis of the MDR1 gene at position 1199 includes hybridization between a nucleic acid sample isolated from the subject and (a) a nucleic acid comprising about 10 to about 100 contiguous nucleotides of a G1199T variant of the nucleotide sequence set forth in SEQ ID NO:5, where the nucleic acid includes at least one of the nucleotide at position 1199 and a base adjacent thereto; or (b) a nucleic acid that is fully complementary to the nucleic acid of (a). In certain variations, the method further includes determining the MDR1 genotype at nucleotide position 1199 of the MDR1 gene. Drugs which are substrates of MDR1 and are amenable to prediction of drug efficacy in subjects in accordance with the above method include cytotoxic chemotherapeutic agents (e.g., doxorubicin, paclitaxol, vinblastine, and vincristine).

It has been demonstrated that the G1199A MDR1 variant (MDR1_(G1199A)) exhibits decreased apical to basolateral transport of HIV protease inhibitors in recombinant epithelial cells, relative to wild-type MDR1 (see Example 3). This effect would have a significant impact on, e.g., transcellular transport when the HIV protease inhibitor is exposed to the apical membrane of epithelial cells (e.g., absorption in the intestine). Determination of the presence or absence of the MDR1_(G1199A) variant can therefore be used to predict the oral bioavailability of HIV protease inhibitors in a subject, and/or penetration of an HIV protease inhibitor into cells and tissues of the lymphoid and central nervous systems of a subject, relative to an individual that is homozygous for the wild-type MDR1 gene.

Accordingly, in yet other variations of the present invention, a method for predicting oral bioavailability and or blood-brain barrier penetration of an HIV protease inhibitor in a subject in need thereof is provided that includes (1) isolating a biological sample from the subject, the biological sample comprising nucleic acids and/or proteins; and (2) analyzing the biological sample to determine at least one of (a) the nucleotide at position 1199 of the MDR1 gene, and (b) the presence or absence of the S400N variant of MDR1 protein; where the presence of A at nucleotide position 1199 or the presence of the S400N variant is indicative of a decreased oral bioavailability or blood-brain barrier penetration of the HIV protease inhibitor in the subject relative an individual that is homozygous for a wild-type MDR1 gene. As indicated above, a particularly suitable technique for analysis of the MDR1 gene at position 1199 includes hybridization between a nucleic acid sample isolated from the subject and (a) a nucleic acid comprising about 10 to about 100 contiguous nucleotides of a G1199A variant of the nucleotide sequence set forth in SEQ ID NO:5, where the nucleic acid includes at least one of the nucleotide at position 1199 and a base adjacent thereto; or (b) a nucleic acid that is fully complementary to the nucleic acid of (a). In certain variations, the method further includes determining the MDR1 genotype at nucleotide position 1199 of the MDR1 gene. HIV protease inhibitors particularly amenable to prediction of oral bioavailability in subjects in accordance with the above method include, for example, amprenavir, indinavir, lopinavir, ritonavir, and saquinavir.

The above methods for predicting responsiveness to a drug or oral bioavailability of an HIV protease inhibitor can further include designing a patient-specific treatment regimen based on the information obtained from using the method. In certain embodiments, a mode of administration for the HIV protease inhibitor can be selected based the information obtained. For example, where a decreased oral bioavailability is indicated for the subject, a clinician may choose, for example, intravenous administration of the HIV protease inhibitor to deliver the drug directly into the bloodstream. Also, a clinician can select an HIV protease inhibitor that is not a substrate for the MDR1. Similarly, where a decreased drug responsiveness is indicated by presence of the MDR1_(G1199T) allele, dosage of the drug can be increased, or a different drug that is not an MDR1 substrate selected for administration.

It has also been demonstrated that cells expressing the G1199A MDR1 polymorphic variant exhibit increased resistance to vinblastine and vincristine relative to cells expressing wild-type MDR1. Accordingly, in other embodiments, a method for predicting the efficacy of vinblastine or vincristine in a subject in need thereof is provided that includes (1) isolating a biological sample from the subject, the biological sample comprising nucleic acids and/or proteins; and (2) analyzing the biological sample to determine at least one of (a) the nucleotide at position 1199 of the MDR1 gene, and (b) the presence or absence of the S400N variant of MDR1 protein; where the presence of A at nucleotide position 1199 or the presence of the S400N variant is indicative of a decreased efficacy of vinblastine or vincristine in the subject relative an individual that is homozygous for a wild-type MDR1 gene. Again, a particularly suitable technique for analysis of the MDR1 gene at position 1199 includes hybridization between a nucleic acid sample isolated from the subject and (a) a nucleic acid comprising about 10 to about 100 contiguous nucleotides of a G1199A variant of the nucleotide sequence set forth in SEQ ID NO:5, where the nucleic acid includes at least one of the nucleotide at position 1199 and a base adjacent thereto; or (b) a nucleic acid that is fully complementary to the nucleic acid of (a). In certain variations, the method further includes determining the MDR1 genotype at nucleotide position 1199 of the MDR1 gene. In still further embodiments, the method further comprises selecting a cytotoxic chemotherapeutic drug for administration to the subject, or determining an appropriate dosage of a chemotherapeutic agent, based on the presence or absence the G1199A MDR1 polymorphism or S400N MDR1 variant (e.g., if the G1199A polymorphism is present, a chemotherapeutic agent which is not vinblastine or vincristine; or selecting an increased dosage of vinblastine or vincristine relative to a subject that does not express the G1199A MDR1 variant).

MDR1 Nucleic Acids, Polypeptides, Recombinant Cells, and Methods of Use

In another aspect, nucleic acids encoding a variant of the MDR1 protein of SEQ ID NO:5, or a fragment thereof, are also provided. In certain embodiments, the MDR1 nucleic acids comprise at least 15, typically at least 17, at least 20, or at least 25, and more typically at least 30, at least 50, or at least 100 or more nucleotides of a variant of the nucleotide sequence set forth in SEQ ID NO:5, where the variant MDR1 nucleic acid includes the nucleotide at least one of the following nucleotide positions: 163, 886, 1199, 1292, 1293, 2814, 3258, and 3271; and where the variant nucleic acid contains a sequence variation (i.e., variant from wild-type) at least one of the foregoing positions. In particular embodiments, the variant MDR1 nucleic acid includes at least one of the following sequence variations: A163C, A886C, G1199T, G1292T, T1293G, T2814G, C3258T, and G3271C. Typically, an MDR1 variant having either the G1292T or T1293G nucleotide substitution will have these substitutions at both positions (herein also referred to as GT1292-3TG or 1292-3TG). Further, in some embodiments in which the MDR1 variant includes a sequence variation at any one or more of positions 163, 886, 1292, 1293, 2814, 3258, and 3271, the variant further includes the G1199A polymorphism.

In some embodiments, the MDR1 variant includes either only one sequence variation at nucleotide position 163, 886, 1199, 2814, 3258, or 3271; or a sequence variation at positions 1292-93 (e.g., the particular sequence variations set forth in Table 1); in yet other embodiments, the MDR1 variant comprises two, three, or more of the foregoing sequence variations. An MDR1 variant of the present invention optionally includes one or more additional sequence variations corresponding to other identified MDR1 polymorphisms (e.g., the particular sequence variations listed in Table 2).

The variant MDR1 nucleic acid can, for example, encode a full-length variant MDR1 protein, or a functional fragment thereof. In yet other embodiments, the MDR1 nucleic acid is a probe or a primer such as, e.g., described above for use in detection of an MDR1 polymorphism in a biological sample.

MDR1 nucleic acids of the present invention further include derivatives encoding other possible codon choices for the same amino acid, or encoding conservative amino acid substitutions thereof. Due to the degeneracy of nucleotide coding sequences, other DNA sequences which encode substantially the same amino acid sequence as a variant MDR1 nucleic acid can be used in the practice of the present invention. These include, but are not limited to, nucleotide sequences comprising all or portions of a variant MDR1 nucleic acid which is altered by the substitution of different codons that encode the same or a functionally equivalent amino acid residue (e.g., a conservative substitution) within the sequence, thus producing a silent change.

MDR1 nucleic acids of the present invention further include those nucleic acids specifically hybridizable or complementary to the foregoing sequences. Hybridizable nucleic acids can comprise sequences complementary to at least 10, 15, 17, 20, 25, 50, 100, 200, or 250 nucleotides or more of a MDR1 gene, including full-length complements of an MDR1 nucleic acid. Nucleic acids are specifically hybridizable to an MDR1 nucleic acid, or to a nucleic acid encoding a MDR1 derivative, under stringent conditions. Stringent conditions are well-known to those of skill in the art and vary predictably depending on conditions such as salt concentrations, temperature, and the base composition of the particular nucleic acid sequence. (See generally Sambrook and Russell, Molecular Cloning, A Laboratory Manual (3rd ed. 2001); Ausubel et al. (eds.), Current Protocols in Molecular Biology (1994); Sambrook et al., Molecular Cloning, A Laboratory Manual (2nd ed. 1989), each incorporated by reference herein.)

Various recombinant DNA methods known in the art can be used to prepare the MDR1 nucleic acids described hereinabove. For example, expression cloning, genomic cloning, and PCR (see, e.g., Russell et al., supra; Sambrook et al., supra; Ausubel et at., supra.) can be used to obtain MDR1 polynucleotides which can be used for further manipulation. Nucleic acid sequences can also be produced by synthesis using standard methods (e.g., by use of a commercially available automated DNA synthesizer) (typically for shorter nucleic acids). Nucleic acids can be further manipulated as desired using routine techniques. (See generally, e.g., Russell et al., supra; Sambrook et al., supra; Ausubel et al., supra.) For example, known MDR1 sequences can be modified to prepare sequences encoding MDR1 variants polypeptides of the present invention using, e.g., standard in vitro site-directed mutagenesis (see, e.g., Hutchison et al., J. Biol. Chem. 253:6551-60 (1978), incorporated by reference herein), the use of TAB® linkers (Pharmacia), PCR mutagenesis methods, and the like.

The MDR1-encoding nucleic acids can be inserted into an appropriate expression vector (i.e., a vector which contains the necessary elements for the transcription and translation of the inserted polypeptide-coding sequence). A variety of host-vector systems can be utilized to express an MDR1 polypeptide-coding sequence. These include, for example, mammalian cell systems transfected with plasmid vectors or infected with virus (e.g., vaccinia virus, adenovirus, parvoviruses (e.g., AAV), sindbis virus, Venezuelan equine encephalitis (VEE) virus, and the like), insect cell systems infected with virus (e.g., baculovirus), microorganisms such as yeast containing yeast vectors, or bacteria transformed with bacteriophage DNA, plasmid DNA, or cosmid DNA. The expression elements of vectors vary in their strengths and specificities. Depending on the host-vector system utilized, any one of a number of suitable transcription and translation elements can be used. In specific embodiments, the MDR1 polypeptide is expressed in human cells, rat cells, other mammalian cells, yeast or bacteria.

Any suitable method can be used for insertion of MDR1 nucleic acids into an expression vector. Suitable expression vectors typically include appropriate transcriptional and translational control signals. Suitable methods include in vitro recombinant DNA and synthetic techniques and in vivo recombination techniques (genetic recombination). Expression of nucleic acid sequences can be regulated by a second nucleic acid sequence so that the encoded nucleic acid is expressed in a host transformed with the recombinant DNA molecule. For example, expression of an MDR1 polypeptide can be controlled by any suitable promoter/enhancer element known in the art. Suitable promoters include, for example, the SV40 early promoter region (Benoist and Chambon, Nature 290:304-10 (1981)), the promoter contained in the 3′ long terminal repeat of Rous sarcoma virus (Yamamoto et al., Cell 22:787-97 (1980)), the herpes thymidine kinase promoter (Wagner et al., Proc. Natl. Acad. Sci. USA 78:1441-45 (1981)), the cytomegalovirus promoter, the translational elongation factor EF-1 promoter, the regulatory sequences of the metallothionein gene (Brinster et al., Nature 296:39-42 (1982)), prokaryotic promoters such as, for example, the lactamase promoter (Villa-Komaroff et al., Proc. Natl. Acad. Sci. USA 75:3727-31 (1978)) or the tac promoter (deBoer et al., Proc. Natl. Acad. Sci. USA 80:21-25 (1983)), plant expression vectors including the cauliflower mosaic virus 35S RNA promoter (Gardner et al., Nucl. Acids Res. 9:2871-88 (1981)), and the promoter of the photosynthetic enzyme ribulose biphosphate carboxylase (Herrera-Estrella et al., Nature 310:115-20 (1984)), promoter elements from yeast or other fungi such as the GAL7 and GAL4 promoters, the ADH (alcohol dehydrogenase) promoter, the PGK (phosphoglycerol kinase) promoter, the alkaline phosphatase promoter, and the like.

Other mammalian promoters include, for example, the following animal transcriptional control regions, which exhibit tissue specificity: the elastase I gene control region which is active in pancreatic acinar cells (Swift et al., Cell 38:639-46 (1984); Omitz et al., Cold Spring Harbor Symp. Quant. Biol. 50:399-409 (1986); MacDonald, Hepatology 7(1 Suppl.):42S-51S (1987)); the insulin gene control region which is active in pancreatic beta cells (Hanahan, Nature 315:115-22 (1985)), the immunoglobulin gene control region which is active in lymphoid cells (Grosschedl et al., Cell 38:647-58 (1984); Adams et al., Nature 318:533-38 (1985); Alexander et al., Mol. Cell. Biol. 7:1436-44 (1987)), the mouse mammary tumor virus control region which is active in testicular, breast, lymphoid and mast cells (Leder et al., Cell 45:485-95 (1986)), the albumin gene control region which is active in liver (Pinkert et al., Genes Dev. 1:268-76 (1987)), the alpha-fetoprotein gene control region which is active in liver (Krumlauf et al., Mol. Cell. Biol. 5:1639-48 (1985); Hammer et al., Science 235:53-58 (1987)); the alpha 1-antitrypsin gene control region which is active in the liver (Kelsey et al., Genes and Devel. 1:161-71 (1987)); the beta-globin gene control region which is active in myeloid cells (Magram et al., Nature 315:338-40 (1985); Kollias et al., Cell 46:89-94 (1986)); the myelin basic protein gene control region which is active in oligodendrocyte cells in the brain (Readhead et al., Cell 48:703-12 (1987)); the myosin light chain-2 gene control region which is active in skeletal muscle (Shani, Nature 314:283-86 (1985)); and the gonadotropic releasing hormone gene control region which is active in the hypothalamus (Mason et al., Science 234:1372-78 (1986)).

In certain embodiments, a vector is used that comprises, in operative combination, a transcription promoter, the MDR1-encoding nucleic acid, a transcription terminator, and one or more origins of replication. In other embodiments, the vector includes one or more selectable markers (e.g., an antibiotic resistance gene). Suitable selectable markers include, for example, those conferring resistance to ampicillin, tetracycline, neomycin, G418, and the like.

Once a suitable expression vector host system and growth conditions are established, methods that are known in the art can be used to propagate it. In addition, host cells can be chosen that modulate the expression of the inserted nucleic acid sequences, or that modify or process the gene product in the specific fashion desired. Expression from certain promoters can be elevated in the presence of certain inducers; thus, expression of the MDR1 sequence can be controlled. Furthermore, different host cells having characteristic and specific mechanisms for the translational and post-translational processing and modification (e.g., glycosylation or phosphorylation) of polypeptides can be used. Appropriate cell lines or host systems can be chosen to ensure the desired modification and processing of the expressed polypeptide. For example, expression in a bacterial system can be used to produce an unglycosylated polypeptide.

The invention further relates to the MDR1 polypeptides encoded by the foregoing MDR1 nucleic acids, including derivatives and analogs. The production and use of MDR1 polypeptides, and derivatives and analogs thereof, are also within the scope of the present invention. MDR1 variant proteins are useful, for example, in studies relating to elucidation of molecular mechanisms of MDR1 function, including, e.g., mechanisms leading to reduction of drug resistance. Such studies can be conducted, for example, using purified and crystallized proteins alone or complexed with particular MDR1 substrates.

Accordingly, in one aspect, the present invention provides an isolated MDR1 protein which is a variant of the wild-type MDR1 protein having the amino acid sequence set forth in SEQ ID NO:4 and having at least one of the following amino acid substitutions: T55P, N296H, S4001, C431L, F938L, and A1091P. In certain variations, the MDR1 protein includes only one amino acid substitution selected from T55P, N296H, S400I, C431L, F938L, and A1091P; in other embodiments, the MDR1 protein includes two or more of the foregoing substitutions. The variant MDR1 proteins of the present invention can optionally include one or more amino acid substitutions corresponding to other known MDR1 polymorphic variants, such as, for example, those set forth in Table 2.

Derivatives of variant MDR1 polypeptides include those altered by substitution, addition, or deletion of one or more amino acid residues that provide for functionally active molecules. MDR1 polypeptide derivatives include, e.g., those containing as a primary amino acid sequence all or part of the amino acid sequence of an MDR1 polypeptide including altered sequences in which one or more functionally equivalent amino acid residues (e.g., a conservative substitution) are substituted for residues within the sequence. In certain embodiments, the MDR1 polypeptide derivative is a functional fragment of a full-length MDR1 variant as described herein having one or more biological activities associated with full-length MDR1 protein.

In a specific embodiment, the MDR1 derivative is a chimeric, or fusion, protein comprising a variant MDR1 polypeptide joined at its amino- or carboxy-terminus via a peptide bond to an amino acid sequence of a different protein. In one embodiment, such a chimeric protein is produced by recombinant expression of a nucleic acid encoding the protein. The chimeric product can be made by ligating the appropriate nucleic acid sequence, encoding the desired amino acid sequences, to each other in the proper coding frame and expressing the chimeric product by methods commonly known in the art. Alternatively, the chimeric product can be made by protein synthetic techniques (e.g., by use of an automated peptide synthesizer).

MDR1 polypeptides can be produced by recombinant DNA techniques, by chemical synthetic methods, or by purification of native polypeptides. MDR1 polypeptides can be isolated and purified by standard methods including chromatography (e.g., ion exchange, affinity, sizing column chromatography, high pressure liquid chromatography), centrifugation, differential solubility, or by any other standard technique for the purification of proteins.

Functional properties of variant MDR1 polypeptides may be evaluated, for example, in vitro in a recombinant cell line expressing the MDR1 variant. Cell lines expressing recombinantly produced MDR1 variants as described herein are also within the scope of the present invention. In typical embodiments, the expression system utilizes a cell type which is amenable to evaluation of transcellular permeability such as, e.g., a polarized epithelial cell line (e.g., LLC-PK1, MDCKII, or Caco-2). In a preferred embodiment, the cell line is stably transformed with a non-viral expression vector (e.g., a pCR3.1 TA vector (Invitrogen, Carlsbad, Calif.) containing cytomegalovirus (CMV) and T7 promoters capable of transcription). A particularly suitable recombinant expression system for expression of functional MDR1 protein is further described hereinbelow (see Examples). In specific embodiments, the recombinant cell line is a recombinant LLC-PK1 cell line expressing MDR1 protein and its variants as set forth in Table 3, infra. Functional properties of MDR1 that can be evaluated using recombinant MDR1-expressing cells include, for example, efflux, uptake, and transcellular transport of agents, including, e.g., pharmaceutical agents.

The recombinant cell lines of the present invention are useful in methods for determining the impact of a MDR1 polymorphism on drug penetration and/or disposition. MDR1-mediated efflux, uptake, or transcellular transport. For example, in certain embodiments, a method is provided for evaluating the influence of a MDR1 polymorphism, or a particular MDR1 haplotype comprising two or more MDR1 polymorphisms, on MDR1-mediated efflux, uptake, or transcellular transport. The method includes (1) contacting a first cell with a therapeutic agent, where the first cell expresses an MDR1 variant having at least one of the following amino acid substitutions: T55P, N296H, S4001, C431L, F938L, and A1091P; (2) contacting a control cell with the therapeutic agent, where the control cell expresses a wild-type MDR1 protein; (3) culturing the first cell and the control cell; (4) determining for each of the first cell and control cell the uptake, efflux, and/or transcellular transport of the therapeutic agent; and (4) comparing the level of uptake, efflux, or transcellular transport of the therapeutic agent in the first cell with that of the control cell. Assays for measuring the uptake, efflux, and/or transcellular transport of an agent in a cell are well-known in the art; exemplary assays are described further infra (see Examples).

In certain variations of the above method for evaluating the influence of one or more MDR1 polymorphisms, the MDR1 protein variant expressed by the first recombinant cell includes only one amino acid substitution selected from T55P, N296H, S400I, C431L, F938L, and A1091P; in other embodiments, the MDR1 protein includes two or more of the foregoing substitutions. The variant MDR1 protein expressed by the first recombinant cell can optionally include one or more amino acid substitutions corresponding to other known MDR1 polymorphic variants, such as, for example, those set forth in Table 2.

Compositions Relating to Rat MRP1

The present invention also provides compositions relating to the rat MRP1 cDNA and protein, polymorphisms within that gene and methods relating to the use of the rat MRP1 for the expression of rat MRP1, and for clinically applicable strategies to identify and adjust the management of medications in patients with exceptional genetic traits, disease conditions, or risk of drug-drug interactions, based on the presence of certain MRP1 polymorphisms. In one aspect, the sequence of the rat MRP1 cDNA is provided. In a related aspect, the sequence of the rat MRP1 protein and nucleic acid sequences encoding that protein are provided.

The rat MRP1 cDNA has been closed and sequenced. (See FIG. 1.) The rat MRP1 sequence exhibits 94.3% and 83.0% nucleotide sequence homology to that of mice and human, respectively (See FIG. 1). The isolated rat MRP1 cDNA, or nucleic acids encoding the rat MRP1 protein or a functional fragment thereof, are also provided.

In certain embodiments, an isolated rat MRP1 nucleic acid is at least 90%, at least 95%, or at least 99% identical to the rat MRP1 sequence of FIG. 1. The terms “identical” or “percent identity,” in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same (i.e., 60% identity, optionally 65%, 70%, 75%, 80%, 85%, 90%, or 95% identity over a specified region), when compared and aligned for maximum correspondence over a comparison window, or designated region, as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. Such sequences are then said to be “substantially identical.” This definition also refers to the complement of a test sequence. Optionally, the identity exists over a region that is at least about 30 amino acids or nucleotides in length, typically over a region that is 50, 75 or 150 amino acids or nucleotides. In one embodiment, the sequences are substantially identical over the entire length of the coding regions.

The terms “similarity,” or “percent similarity,” in the context of two or more polypeptide sequences, refer to two or more sequences or subsequences that have a specified percentage of amino acid residues that are either the same or similar as defined in the conservative amino acid substitutions defined above (i.e., at least 60%, optionally 65%, 70%, 75%, 80%, 85%, 90%, or 95% similar over a specified region), when compared and aligned for maximum correspondence over a comparison window, or designated region as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. Such sequences are then said to be “substantially similar.” Optionally, this identity exists over a region that is at least about 25 amino acids in length, or more preferably over a region that is at least about 50, 75 or 100 amino acids in length.

For sequence comparison, typically one sequence acts as a reference sequence to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are typically input into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. The sequence comparison algorithm then calculates the percent sequence identity for the test sequence(s) relative to the reference sequence, based on the designated program parameters.

Optimal alignment of sequences for comparison can be conducted, for example, by the local homology algorithm of Smith and Waterman (Adv. Appl. Math. 2:482 (1981)), by the homology alignment algorithm of Needleman and Wunsch (J. Mol. Biol. 48:443 (1970)), by the search for identity method of Pearson and Lipman (Proc. Natl. Acad. Sci. USA 85:2444 (1988)), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by visual inspection (see, generally Ausubel et al., Current Protocols in Molecular Biology, John Wiley and Sons, New York (1996)).

One example of a useful algorithm is PILEUP. PILEUP creates a multiple sequence alignment from a group of related sequences using progressive, pairwise alignments to show relationship and percent sequence identity. It also plots a tree or dendogram showing the clustering relationships used to create the alignment. PILEUP uses a simplification of the progressive alignment method of Feng and Doolittle (J. Mol. Evol. 35:351-60 (1987)). The method used is similar to the CLUSTAL method described by Higgins and Sharp (Gene 73:237-44 (1988); CABIOS 5:151-53 (1989)). The program can align up to 300 sequences, each of a maximum length of 5,000 nucleotides or amino acids. The multiple alignment procedure begins with the pairwise alignment of the two most similar sequences, producing a cluster of two aligned sequences. This cluster is then aligned to the next most related sequence or cluster of aligned sequences. Two clusters of sequences are aligned by a simple extension of the pairwise alignment of two individual sequences. The final alignment is achieved by a series of progressive, pairwise alignments. The program is run by designating specific sequences and their amino acid or nucleotide coordinates for regions of sequence comparison and by designating the program parameters. For example, a reference sequence can be compared to other test sequences to determine the percent sequence identity relationship using the following parameters: default gap weight (3.00), default gap length weight (0.10), and weighted end gaps.

Another example of an algorithm that is suitable for determining percent sequence identity and sequence similarity is the BLAST algorithm, which is described in Altschul et al. (J. Mol. Biol. 215:403-10 (1990)). Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information <http://www.ncbi.nlm.nih.gov/>. This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al., supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a wordlength (W) of 11, an expectation (E) of 10, a cutoff of 100, M=5, N=−4, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a wordlength (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff and Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)).

In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin and Altschul, Proc. Natl. Acad. Sci. USA 90:5873-87 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is typically between about 0.35 and about 0.1. Another indication that two nucleic acids are substantially identical is that the two molecules hybridize to each other under stringent conditions. The phrase “hybridizing specifically to” refers to the binding, duplexing, or hybridizing of a molecule only to a particular nucleotide sequence under stringent conditions when that sequence is present in a complex mixture (e.g., total cellular) DNA or RNA. “Bind(s) substantially” refers to complementary hybridization between a probe nucleic acid and a target nucleic acid and embraces minor mismatches that can be accommodated by reducing the stringency of the hybridization media to achieve the desired detection of the target polynucleotide sequence.

“Stringent hybridization conditions” and “stringent hybridization wash conditions” in the context of nucleic acid hybridization experiments, such as Southern and Northern hybridizations, are sequence-dependent, and are different under different environmental parameters. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen, Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes, part I, chapter 2 “Overview of Principles of Hybridization and the Strategy of Nucleic Acid Probe Assays,” Elsevier, N.Y. (1993). Generally, highly stringent hybridization and wash conditions are selected to be about 5° C. lower than the thermal melting point (Tm) for the specific sequence at a defined ionic strength and pH. Typically, under “stringent conditions,” a probe will hybridize to its target subsequence, but to no other sequences.

The Tm is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Very stringent conditions are selected to be equal to the Tm for a particular probe. An example of stringent hybridization conditions for hybridization of complementary nucleic acids which have more than 100 complementary residues on a filter in a Southern or Northern blot is 50% formamide in 4-6×SSC or SSPE at 42° C., or 65-68° C. in aqueous solution containing 4-6×SSC or SSPE. An example of highly stringent wash conditions is 0.15 M NaCl at 72° C. for about 15 minutes. An example of stringent wash conditions is a 0.2×SSC wash at 65° C. for 15 minutes. (See generally Sambrook et al., Molecular Cloning, A Laboratory Manual, 2nd ed., Cold Spring Harbor Publish., Cold Spring Harbor, N.Y. (1989)). Often, a high stringency wash is preceded by a low stringency wash to remove background probe signal.

In another aspect, the rat MRP1 cDNA or nucleic acid is used to develop a cell line expressing recombinant rMRP1. A rat MRP1-expressing cell line is typically a stable cell line or a stable epithelial cell line. Cell lines expressing recombinant rMRP1 protein can be used for trans-epithelial transport studies of drugs, prodrugs and other substances. With a parallel human MRP1 continuous cell line developed, the role of rMPR1 and human MRP1 in mediating resistance to and modifying the drug disposition profile of commonly prescribed anti-tumor drugs can be elucidated.

Cell Lines Expressing MRP1 or MDR1 and Related Methods

In yet another aspect of the present invention, a mammalian MPR1 or MDR1 nucleic acid is used to develop a cell line expressing a recombinant MRP1 or MDR1 protein. The MRP1 or MDR1 nucleic acid can be rat, human, or other mammal. Methods for expression of a recombinant protein in host cells are generally known in the art. Exemplary methods that may be used for expression of recombinant MRP1 or MDR1 are described supra with respect to MDR1 protein variants. Particularly suitable cell lines are those which are capable of transcellular transport, typically, for example, a polarized cell line or cell line capable of differentiation to a polarized phenotype (e.g., an epithelial cell line). In a preferred embodiment, the cell line stably expresses the recombinant protein. In exemplary embodiments, the cell line is a stable epithelial cell line such as, for example, an intestinal or renal epithelial cell line (e.g., LLC-PK1, MDCKII, or Caco-2). As used herein, a “stable” cell line contains and expresses the recombinant nucleic over many generations. In typical variations, a non-viral vector comprising the MRP1 or MDR1 nucleic acid is used to develop the MRP1 or MDR1-expressing cell line. In one exemplary embodiment, the non-viral vector is a pCR3.1 TA vector (Invitrogen, Carlsbad, Calif.) containing cytomegalovirus (CMV) and T7 promoters capable of transcription (see, e.g., Yang et al., Biotechniques 33:196, 8, 200 passim. (2002)). The use of a non-viral vector facilitates long-term expression of the MRP1 or MDR1 recombinant protein without the co-expression of viral elements, which may influence MRP1 or MDR1 function and may therefore, for example, influence the ability to detect functional variability due to MRP1 or MDR1 polymorphisms.

The MRP1- and MDR1-expressing cell lines of the present invention are useful, for example, for evaluating the influence of an MRP1 or MDR1 protein to the disposition of an agent in cells, and/or penetration of an agent across absorption barriers. For example, in certain embodiments, a method for evaluating the contribution of an MRP1 or MDR1 protein to the disposition or penetration of an agent is provided that includes contacting the cell line expressing the MRP1 or MDR1 protein with the agent; culturing the contacted cell line; and determining at least one of the uptake, efflux, or transcellular transport of the agent. The study of disposition or penetration of agents in vitro using the cell lines and methods described herein is useful for predicting pharmacokinetics in vivo. The screening of candidate therapeutic agents using the cell lines and methods of the present invention can facilitate the development of drugs that can, for example, efficiently penetrate absorption barriers (e.g., intestine and blood-brain barrier) as well as efficiently accumulate in target cells and tissues.

For example, a cell line expressing a recombinant mammalian MRP1 or MDR1 protein can be used to determine a drug disposition profile of commonly prescribed anti-tumor drugs. In particular, a cell line expressing a recombinant mammalian MRP1 or MDR1 protein can be contacted with a therapeutic agent to determine the susceptibility, toxicity, LD₅₀, or the like of the therapeutic agent on the cell line. The cell line can express one or more MRP1 and/or MDR1 or other related (i.e., a homolog or ortholog) transporter proteins.

In particular embodiments, the cell lines of the present invention are used to determine the influence of a MDR1 or MRP1 polymorphism, or a particular MDR1 or MRP1 haplotype having a combination of two or more polymorphisms, on MDR1- or MRP1-mediated efflux, uptake, or transcellular transport. In certain embodiments, the method includes (1) contacting a first cell expressing a recombinant MRP1 or MDR1 protein with a therapeutic agent, where the recombinant MRP1 or MDR1 protein expressed in the first cell is an allelic variant of a wild-type MRP1 or MDR1 protein; (2) contacting a control cell with the therapeutic agent, where said control cell expresses the wild type MRP1 or MDR1 protein corresponding to the allelic variant; (3) culturing the first cell and control cell; (4) determining for each of the first cell and control cell at least one of the uptake, efflux, or transcellular transport of the therapeutic agent; and (5) comparing the level of uptake, efflux, or transcellular transport of the therapeutic agent in the first cell with that of the control cell. Cell lines which are particularly suitable for use in the above method include, for example, the recombinant MDR1-expressing cell lines set forth in Table 3, infra.

In exemplary embodiments of the present invention, DNA plasmid vectors capable of expressing MRP1 transcripts in vitro have been constructed. The same set of vectors are designed to allow generation of stable recombinant mammalian cells. Suitable host cells include, for example, LLC-PK1 (a porcine epithelial cells that form tight junctions), MDCK (Madin-Darby canine kidney epithelial cells that form tight junctions), and K562 (human leukemia cell). These stable recombinant cells can be stored in liquid N₂ freezers and passaged in culture. A list of these recombinant cell lines expressing genetic variants of Pgp, including wild-type control (Table 2) are listed in Table 3.

TABLE 2 A partial list of MDR1 genetic variants that have been linked, or proposed to modulate Pgp function in vitro or in vivo. Allelic Nucleotide Frequency Position Location Effect (%)† Reference G→A exon 11 Ser→Asn 5.5 {Cascorbi, 2001 #5; at 1199 At 400 Hoffmeyer, 2000 #6; Kim, 2001 #8} C→T exon 12 wobble 41.0 {Kioka, 1989 #95; at 1236 Cascorbi, 2001 #5; Hoffmeyer, 2000 #6; Kim, 2001 #8} G→T exon 21 Ala→Ser 41.6 {Mickley, 1998 #33; at 2677 At 893 Kioka, 1989 #95; Cascorbi, 2001 #5; Kim, 2001 #8} G→A exon 21 Ala→Thr 1.9 {Mickley, 1998 #33; at 2677 At 893 Kioka, 1989 #95; Cascorbi, 2001 #5; Kim, 2001 #8} C→T exon 26 Wobble 53.9 {Kioka, 1989 #95; at 3435 (proposed to Cascorbi, 2001 #5; be linked Hoffmeyer, 2000 #6; to 1236 and Kim, 2001 #8} 2677 in producing functional effects) †Allelic frequencies are reported as the frequency of the polymorphism in a Caucasian population.

TABLE 3 A list of novel, stable recombinant-epithelial cell lines expressing Pgp and genetic variants. Recombinant LLC-PKI cell Unique MDR1 coding sequence of expressing MDR1 Pgp protein expressed protein and it Nucleotide variants (validated) Amino acid Comments Wild type None None Stable cell lines with MDR1 sequence and function validated MDRI_(1199A) C→A at 1199 Ser→Asn at Reduced substrate transport 400 activity; altered cancer drug resistance MDR1_(1199T) C→T at 1199 Ser→Ile at 400 Altered efflux transport activity of MDR1 and drug resistance MDR1_(163C) A→C at 163 Thr→Pro at 55 MDR1_(1292-3TG) GT→TG at Cys→Leu at 1292-3 431 MDR1_(2814G) T→G at 2914 Phe→Leu at 938 MDR1_(3271C) G→C at 3271 Ala→Pro at 1091 MDR1_(2677T) G→T at 2677 Ala→Ser at 893 MDR1_(2677A) G→A at 2677 Ala→Thr at 893 MDR1_(1236T) C→T at 1236 No aa change at 312 MDR1_(3435T) C→T at 3435 No aa change at 1145 MDR1_(1236T/2677T) C→T at 1236 No aa change at (double variant) G→T at 2677 312 Ala→Ser at 893 MDR1_(1236T/2677T/3435T) C→T at 1236 No aa change at (triple variant) G→T at 2677 312 C→T at 3435 Ala→Ser at 893 No aa change at 1145 MDR1_(2677T/3435T) G→T at 2677 Ala→Ser at 893 (double variant) C→T at 3435 No aa change at 1145 MDR1_(3258T) C→T at 3258 No aa change at 1087

In an exemplary embodiment, to demonstrate utility of recombinant epithelial expressing MDR1 or MRP1, a cell-line that express genetic variant, G1199A (Tables 1-2) of MDR1 has been constructed. The examples (infra) describe the methods for isolating such a cell line and exemplary data that validated the function of stable recombinant cells and utility in evaluation of drug resistance, as well as in effects of genetic variations in drug transport with LLC-PK1 cell expressing G1199A MDR1 variant (referred to as MDR1₁₁₉₉). The efflux transport of Rhodamine-123 in MDR1₁₁₉₉ cells was approximately 4.5-fold lower than efflux in MDR1_(wt) (wild-type control) cells. Cytotoxicity studies have shown that MDR1_(wt) and MDR1₁₁₉₉ cells exhibited similar resistance to doxorubicin; however, MDR1₁₁₉₉ cells were more resistant to vinblastine and vincristine. The apparent transepithelial permeability ratios of R123 in MDR1_(wt) and MDR1₁₁₉₉ cells were 3.54±0.94 and 2.02±0.51 (p<0.05), respectively. Therefore, the G1199A polymorphism alters the efflux and transepithelial permeability of a fluorescent substrate and sensitivity to select cytotoxic agents, which may influence drug disposition and therapeutic efficacy of some P-glycoprotein substrates.

Employing a similar technology, continuous MRP1 cell lines have been constructed. These recombinant cells were derived from MDCK-II epithelial cells that form tight junctions, suitable for transepithelial transport assessment. Several stable clones of epithelial cells that consistently express rMRP1 were constructed, which could be differentiated to form a polarized monolayer for transepithelial flux studies. These cells were shown express full-length transcript. The cells that consistently over-expressed rMRP1 were expanded from a single cell clone and analyzed for MRP1 function. These MRP1-expressed cells exhibited consistent efflux transport of an MRP1 fluorescence substrate calcein, as evident by 5.2- to 8.7-fold higher efflux activity over 17 weeks in culture. In addition, the efflux transport activity is specifically mediated by MRP1, because addition of MRP1 inhibitors, indomethacin (100 μM) or MK571 (10 μM, an MRP1-specific inhibitor), suppressed efflux transport of calcein back to the control level (See manuscript 1).

Employing the recombinant cells expressing rat or human MRP1, it was demonstrated that rMRP1 has a similar substrate selectivity compared to human MRP1, and could confer drug resistance to cells. The differentiated recombinant rMRP1-MDCK cells exhibit directional transepithelial transport of vinblastine. These stable recombinant cells can be used to evaluate drug candidates that are either MRP1 substrates or inhibitors, as well as further elucidate MRP1 structure and function.

In further embodiments, recombinant cell line are provided based on: (1) other types of epithelial cells to provide closer prediction of tissue barriers controlling the drug penetration, availability and elimination, (2) expression vectors containing more than one genetic variation (allele or polymorphism) of transporter proteins, including the two described herein (MDR1 and MRP1), (3) continuous cell lines expressing multiple transport proteins to estimate the combination effects, and (4) modifications of plasmid sequences to enhance expression, localization and function.

Based on the sequence variations described herein, therapeutic strategies such as single chain inhibitory RNA or DNA sequences, antisense RNA or DNA sequences, and sequence specific ribozymes designed to interfere with drug resistance and transport function of MDR1 and MRP1 can be constructed. Also, these sequences can be used to identify individuals carrying these traits to manage therapeutic efficacy and potential toxicity due to altered drug delivery and disposition. The plasmid vectors, containing for example the polymorphisms described in Tables 1-3, can also be used to produce RNA standards to estimate the number of RNA transcripts in tumor cells or other surrogate cells. The availability of RNA standards allows a quantitative estimate of transcript copy number, instead of the highly variable semiquantitative numbers, based on so called house keeping gene expression. House keeping gene expression can be variable for the same subject as well as among human subjects.

While there is currently no FDA regulatory guidance in place that require in vitro assessment of susceptibility to drug transporters for filing a new drug application, many pharmaceutical companies anticipate that such data would be required in a near future. At present, there are no validated continuous cell lines that consistently express MRP1 and MDR1. Drug transport studies are often done with epithelial cells such as Caco-2, colon epithelial cells as a part of general, membrane permeability studies. Such studies only provide very limited information on the role of MRP1 and MDR1, due to the data only available from inhibitory effects. Details such as velocity and rate constant of new drug candidates and their derivatives require availability of recombinant cells such as those described here. The continuous cell lines according to the present invention can provide a standard for routine evaluation of new compounds to identify new drug candidates with most desirable pharmaceutical qualities. These drug qualities include high degree of tissue penetration, oral absorption, low metabolism and toxicity.

To manage potential drug interactions, the US FDA now requires data on the drug candidate's susceptibility to CYP450 enzymes as a part of preclinical characterization. The FDA guidance was a culmination of (1) availability of purified enzymes, (2) well-characterized in vitro methods, and (3) establishment of in vitro and in vitro correlates of key enzymes such as cytochrome P450 isozymes (e.g., CYP2D6, CYP3A, CYP2C9, CYP2C19). As cells that over expressed MDR1 and MRP1 become widely available for drug screening, evaluation of this membrane bound transporter function is expected to be standardized. The cell lines described in this invention provide such standards to accelerate identification of new compounds with desirable pharmaceutical properties and serve as a tool to predict drug penetration, absorption, distribution, elimination and bioavailability.

Another clinical use of the current invention is to develop immunodiagnostic assays for MRP2 and MDR1. Specific antibodies to these proteins can be prepared using as immunogens either the intact proteins themselves or specific peptide sequences therefrom. Such assays can be used by clinicians to assess, for example, the likelihood that a particular chemotherapeutic agent or drug regimen would be effective against the tumor. An assay would be performed by taking a biopsy from the target tumor, incubating it in vitro with labeled anti-MRP2 and/or anti-MDR1 and evaluating the immunohistochemical results.

The following examples are provided merely as illustrative of various aspects of the invention and shall not be construed to limit the invention in any way.

EXAMPLE 1 Impact of G1199A on Pgp-Mediated Efflux Transport Activity

In this example, the significance of the MDR1 G1199A polymorphism, resulting in a Ser400Asn modification in P-glycoprotein is elucidated. Stable recombinant LLC-PK1 epithelial cells expressing either MDR1_(wt) or MDR1₁₁₉₉ have been developed to evaluate functional consequences of G1199A. Comparable expression of mRNA and protein in the MDR1-expressed cells and correct localization of P-glycoprotein in the apical membrane of recombinant cells was verified. The efflux transport of Rhodamine-123 in MDR1₁₁₉₉ cells was approximately 4.5-fold lower than efflux in MDR1_(wt) cells. Cytotoxicity studies have shown that MDR1_(wt) and MDR1₁₁₉₉ cells exhibited similar resistance to doxorubicin; however, MDR1₁₁₉₉ cells were more resistant to vinblastine and vincristine. The apparent transepithelial permeability ratios of R123 in MDR1_(wt) and MDR1₁₁₉₉ cells were 3.54±0.94 and 2.02±0.51 (p<0.05), respectively. Therefore, the G1199A polymorphism alters the efflux and transepithelial permeability of a fluorescent substrate and sensitivity to select cytotoxic agents, which may influence drug disposition and therapeutic efficacy of some P-glycoprotein substrates.

Introduction

The human multidrug resistance gene (MDR1) encodes a 170 kilodalton integral membrane protein that mediates ATP-dependent substrate efflux. The protein product, P-glycoprotein, a member of the ATP-binding cassette (ABC) superfamily of transporters, resides in the plasma membrane and functions as an efflux transporter of a wide variety of natural compounds and lipophilic xenobiotics (reviewed in Lin et al., Adv. Drug. Deliv. Rev. 55:53-81 (2003)). While the contribution of P-glycoprotein in multidrug resistance for cancer chemotherapy is well documented, the role of P-glycoprotein in drug disposition is not fully understood and has continued to generate significant debate. P-glycoprotein mediates the energy-dependent efflux of a broad range of xenobiotics in epithelial tissues throughout the human body including the intestinal mucosa, liver canalicular membrane, kidney proximal tubules, blood-brain-barrier, and placenta (Schinkel, Semin. Cancer Biol. 8:161-70 (1997)). P-glycoprotein efflux may, therefore, act to decrease intestinal absorption, enhance biliary excretion and renal tubular secretion, and limit drug distribution to the fetus and brain. Because P-glycoprotein is found in tissues important in drug disposition, variation in expression and function of P-glycoprotein due to genetic polymorphisms of MDR1 may influence pharmacokinetics and, in turn, pharmacodynamics.

Recent progress has been made in identifying genetic polymorphisms in the MDR1 gene in normal human tissues. The first major screen of the MDR1 gene in 188 healthy Caucasian subjects, identified 15 single nucleotide polymorphisms; however, only one, a C→T transition at nucleotide position 3435 (C3435T), was shown to correlate with decreased intestinal P-glycoprotein expression and digoxin exposure in vivo (Hoffmeyer et al., Proc. Natl. Acad. Sci. USA 97(7):3473-8 (2000)). As the C3435T polymorphism in exon 26 is a synonymous polymorphism that does not modify the amino acid sequence of P-glycoprotein, several investigators have searched for clues to the significance of C3435T. Another study reported that C3435T is linked to a non-synonymous G2677T polymorphism, resulting in an alanine to serine transition at amino acid 893, and another synonymous SNP, C1236T (Kim et al., Clin. Pharmacol. Ther. 70(2):189-99 (2001)). Several other studies have attempted to link MDR1 polymorphisms, particularly C3435T, to changes in P-glycoprotein expression and function, and subsequent changes in drug disposition profiles. However, much of the data has been contradictory and inconclusive as to the influence of MDR1SNPs and haplotypes on P-glycoprotein expression and in vivo drug disposition. A systematic study, designed to address genetic variants at the cellular and molecular level, is needed to define the functional significance of and the linkage between genetic polymorphisms of MDR1 and their impact on clinical pharmacokinetics.

Therefore, a recombinant expression system in epithelial cells capable of expressing P-glycoprotein variants in a reproducible manner to systematically study the influence of genetic polymorphisms in MDR1 has been developed. Thus far, in vitro studies in the literature to evaluate the influence of MDR1 polymorphisms on P-glycoprotein efflux have produced variable results. Discrepancies exist in the reported influence of G2677T and C3435T on P-glycoprotein activity in various vector and expression systems (Kim et al., Clin. Pharmacol. Ther. 70(2):189-99 (2001); Kimchi-Sarfaty et al., Mol. Pharmacol. 62:1-6 (2002); Morita et al., Biochem. Pharmacol. 65:1843-52 (2003)). In addition, some transient expression systems used to evaluate changes in activity cannot be used to evaluate transepithelial transport, which is important in assessing drug uptake and permeability. Because linkage to C1236T and G2677T complicates the influence of C3435T, the functional role of the G1199A polymorphism has been evaluated as a model for recombinant P-glycoprotein expression system. MDR1G1199A, which results in a serine to asparagine transition at amino acid 400 in a cytoplasmic domain of P-glycoprotein, has a reported allelic frequency of approximately 5.5% in Caucasians (Hoffmeyer et al., Proc. Natl. Acad. Sci. USA 97(7):3473-8 (2000); Kim et al., Clin. Pharmacol. Ther. 70(2):189-99 (2001); Cascorbi et al., Clin, Pharmacol. Ther. 69(3):169-74 (2001)). Thus far, G1199A has not been shown to be linked to other SNPs. The goal of this study was to develop a recombinant MDR1 expression system to evaluate the functional significance of G1199A in epithelial cells. The G1199A polymorphism was found to alter efflux and transepithelial transport as well as altering the sensitivity profiles of cells to cytotoxic drugs.

Methods

Cell Culture

LLC-PK1 control and transfected cells were grown in complete media consisting of RPMI medium 1640 (Invitrogen, Carlsbad, Calif.) supplemented with 10% (v/v) fetal calf serum and 1% (v/v) antibiotic-antimycotic and grown at 37° C. in the presence of 5% CO₂. For deconvolution immunofluorescent microscopy and transepithelial transport studies, cells were grown in Medium 199 (Invitrogen) supplemented with L-glutamine, 10% (v/v) fetal calf serum, and 1% (v/v) antibiotic-antimycotic and grown at 37° C. in the presence of 5% CO₂.

Generation of MDR1_(wt) and MDR1₁₁₉₉ Plasmids

Total RNA was extracted from the MDR1-overexpressing cell line, MES-SA-DX5 and full-length MDR1 cDNA was generated using the high-fidelity protocol developed previously (Yang et al., Biotechniques 33:196, 8, 200 passim. (2002)). The isolated MDR1 cDNA was cloned into a linearized pCR3.1 TA vector (Invitrogen, Carlsbad, Calif.) containing cytomegalovirus (CMV) and T7 promoters capable of transcription also described previously (Yang et al., supra). The plasmid stock was designated as MDR1_(wild-type) (MDR1_(wt)).

The plasmid containing the G1199A polymorphism was generated by subcloning a fragment containing the polymorphism into the MDR1_(wt) plasmid utilizing the restriction enzyme, HindIII. This strategy employed two PCR steps to insert the 1199A variant and is further detailed in FIG. 1. Two pairs of primers were created. The first pair overlapped the G1199A region with the 1199A variation in the middle of the oligonucleotide: 5′CAGAAATGTTCACTTCAATTACCCATCTCGAAAAG 3′ (residues 1182-1216; forward primer) (SEQ ID NO:6) and 5′ TTTCGAGATGGGTAATTGAAGTGAACATTTCTG 3′ (residues 1182-1214; reverse primer) (SEQ ID NO:7). The other set of primers overlapped the restriction enzyme sites for HindIII, one recognition site in the pCR3.1 vector, 5′ GTTTAACTTAAGCTTGGTACC 3′ (residues-697-674; forward primer) (SEQ ID NO:8), and one site in MDR1 cDNA, 5′ GGTACTAAGCTTTCTGTCTTGG 3′ (residues 2031-2052; reverse primer) (SEQ ID NO:9). The first PCR step generated two fragments containing 1199A at one end of the fragment and the HindIII site at the other end. The two fragments were annealed together and amplified using the HindIII primers. The amplified PCR fragment and the pCR3.1-hMDR1 plasmid were digested with HindIII and annealed together. Clones were screened by restriction enzyme mapping with PstI and EcoRI and the sequence was verified using Big-Dye 3.0 chemistry (Applied Biosystems, Foster City, Calif.) and an ABI Prism 377 DNA Sequencer (Applied Biosystems). The variant plasmid stock was designated MDR1₁₁₉₉.

Isolation of Stable Recombinant Cells Expressing MDR1_(wt) or MDR1₁₁₉₉

The MDR1_(wt) and MDR1₁₁₉₉ plasmids were transfected into the porcine kidney epithelial cell line, LLC-PK1. Ten million cells were resuspended in complete media and transferred to a 0.4 cm electroporation cuvette (Bio-Rad Laboratories, Hercules, Calif.) with 10 μg of plasmid DNA (either MDR1_(wt) or MDR1₁₁₉₉). Electroporation was performed at 230 V and 975 μF in a Gene Pulser® II (Bio-Rad Laboratories). Cells were placed on ice for 10 min and cultured in complete media for selection. Transfected LLC-PK1 cells with the highest level of P-glycoprotein expression were selected to generate stable cell lines expressing MDR1_(wt) or MDR1₁₁₉₉. Cells were treated for 3 days with 300 μg/mL G418, a neomycin derivative (Calbiochem, San Diego, Calif.). A fraction of the cells were briefly exposed for another 3 days to 5 μM doxorubicin (VHA PLUS®, Irving, Tex.), a cytotoxic P-glycoprotein substrate, and continued selection by G418 pressure. The cells expressing high levels of P-glycoprotein were further expanded and verified to have stable expression allowing for functional evaluation.

Characterization of P-Glycoprotein Expression

Absolute quantitation of MDR1 mRNA transcripts in cellular samples was performed using the ABI Prism 7900HT Sequence Detection System (Applied Biosystems). An MDR1RNA standard was generated by the method described previously, and concentration was measured by absorbance at 260 nm and converted to the number of copies of MDR1RNA by the molecular weight (Yang et al., Biotechniques 33:196, 8, 200 passim. (2002)). A dilution series of the MDR1RNA standard was used to generate a standard curve of the number of copies of MDR1 mRNA vs. C_(T) value. Following RNA isolation, 100 ng of total RNA of each cellular sample was analyzed in triplicate to obtain a C_(T) value and estimation of the number of copies of MDR1 mRNA.

Immunoblot analysis was used to detect protein expression; briefly, 2×10⁶ cells were pelleted and lysed in a lysis buffer containing SDS and β-mercaptoethanol. Protein concentration was measured by a microplate assay protocol (Bio-Rad Laboratories). Electrophoresis and transfer was run according to instructions for Mini-PROTEAN® II Electrophoresis Cell and Mini Trans-Blot® Electrophoretic Transfer Cell (Bio-Rad Laboratories). Nitrocellulose blots were blocked with 5% evaporated milk in TTBS buffer (0.1% Tween 20; 20 mM Tris-HCl, 0.9% NaCl, pH 7.6). Immunoblotting was performed with the anti-P-glycoprotein monoclonal antibody F4 (Sigma, St. Louis, Mo.) followed by a secondary HRP-conjugated goat anti-mouse IgG. ECL reagents were used as a substrate and blots were exposed to X-ray film for visualization of the protein bands.

For deconvolution immunofluorescent microscopy, 4×10⁵ cells were plated on glass chamber slides (8-well; Lab-Tek™; Nalge Nunc International, Rochester, N.Y.) and grown for 4 days. Cells were fixed with 2% paraformaldehyde in phosphate buffered saline (PBS) (pH 7.4) and permeabilized with 0.2% Triton X-100. Blocking was performed with 2% goat serum in PBS containing 1% BSA (PBS/BSA). Slides were immunostained with anti-P-glycoprotein monoclonal antibodies F4 (Sigma) and C219 (Calbiochem) followed by goat anti-mouse secondary antibody, Alexa-594 (Molecular Probes, Eugene, Oreg.). Slides were mounted with Fluoromount-G (Southern Biotechnology, Birmingham, Ala.) and images were collected with a Zeiss Axiovert 200 MAT deconvolution microscope (Carl Zeiss, Oberkochen, Baden-Wuerttemberg, Germany) with a 63× oil immersion objective lens, and analysis was performed with SlideBook 3.0® software (Intelligent Imaging Innovation, Denver, Colo.).

Intracellular Accumulation Assay

One million cells per well were plated overnight on 6-well plates (Corning, Corning, N.Y.). Cells were washed with PBS and incubated for 90 min in 5 μM Rhodamine-123 (R123) (Sigma) in serum-free RPMI medium 1640. Cells were again washed with PBS and then allowed to efflux for 60 min in complete media. After efflux, cells were trypsinized, washed, and resuspended in 1% paraformaldehyde. Cells were analyzed with a FACScan flow cytometer (Becton Dickinson, Franklin Lakes, N.J.) using CellQuest software (Becton Dickinson). Ten thousand cells from each sample were analyzed for forward scatter, side scatter, and R123 accumulation. Inhibition was performed at 1 μM GF120918.

Cytotoxic Drug Sensitivity Assay

LLC-PK1 control and MDR1 recombinant cells were plated at a density of 1×10⁴ cells/well in a 96-well plate in complete media and allowed to attach for 4 hours at 37° C. Varying concentrations of doxorubicin, vinblastine (Bedford Laboratories, Bedford, Ohio), and vincristine (Faulding, Paramus, N.J.) were added to the cells in quadruplicate. Cells were incubated with the cytotoxic drugs for 3 days at 37° C. On the second day of the incubation, 1 μCi of ³H-thymidine (specific activity: 81.1 Ci/mmole) was added to each well and incubated overnight. Cells were harvested on day 3 with a cell harvester.

Transepithelial Transport Assay

LLC-PK1 control and recombinant MDR1 cells were plated at a density of 2×10⁶ cells/24 mm well on permeable supports (Transwell™; 3.0 μm membrane pore size; Corning) and grown for 4 days; media was refreshed after two days in culture. Fresh media was added to the cells one hour before the initiation of the experiment, and transepithelial electrical resistance (TEER) values were measured with a Millicell®-ERS (Millipore, Billerica, Mass.). For transport of R123 across the epithelial cells, 1 μM R123 in Opti-MEM (Invitrogen) was added to either the apical or basolateral compartments with fresh Opti-MEM medium added to the opposite side. Inhibition was performed at 1 μM GF120918. Aliquots of 50 μL were taken from apical and basal compartments at 1, 2, 3, and 4 hours. Fluorescence intensity of R123 was measured with a Gemini XS microplate spectrofluorometer (Molecular Devices, Sunnyvale, Calif.) with SoftMax Pro® software (Molecular Devices); excitation was set at 488 nm and emission at 525 nm. Apparent permeability (P_(app)) was calculated in the apical-to-basolateral direction (P_(appA→B)) and in the basolateral-to-apical direction (P_(appB→A)) as described by Polli et al. Briefly, P_(app)=(1/(A*C₀))*(dQ/dt), where A is the surface area of permeable support, C₀ is the initial concentration in the donor compartment, and dQ/dt is the rate of transfer of compound into the acceptor compartment. The ratio of P_(appB→As/P) _(appA→B) was also estimated to evaluate P-glycoprotein-mediated directional efflux.

Results Generation of Cells Expressing Recombinant MDR1_(wt) or MDR1₁₁₉₉

MDR1 vectors were successfully constructed for mammalian cell expression containing either the wild-type or variant allele at nucleotide position 1199. The final plasmid constructs were verified by directly sequencing the MDR1 insert. The introduction of the 1199A polymorphism from a wild-type plasmid was based on a restriction enzyme approach, and is described in FIG. 3. These plasmids, referred to as MDR1_(wt) and MDR1₁₁₉₉ were used for generating stable recombinant cells for functional studies.

LLC-PK1 cells were selected as the host to isolate stable epithelial cells for continuous expression of MDR1. These cells differentiate to form polarized monolayers for transepithelial efflux studies. As mammalian cells, LLC-PK1 should provide proper post-translational modification, which has been demonstrated to play a role in P-glycoprotein function (Loo et al., J. Biol. Chem. 268:19965-72 (1993); Loo et al., J. Biol. Chem. 268:3143-9 (1993); Loo et al., J. Biol. Chem. 269:7243-8 (1994); Loo et al., Biochem. 33:14049-57 (1994)). To further reduce the baseline levels of P-glycoprotein-like activity in these cells, a line of LLC-PK1 cells was selected and cloned that was P-glycoprotein negative based on its retention of R123 and sensitivity to the cytotoxic P-glycoprotein substrate doxorubicin. P-glycoprotein-deficient LLC-PK1 cells were transfected with respective plasmids by electroporation, and systematically selected for high levels of P-glycoprotein expression. The recombinant cells expressing MDR1_(wt) or MDR1₁₁₉₉ in a stable and consistent manner were cloned and expanded for functional studies. Direct sequencing was performed to verify expression of the correct allele at nucleotide position 1199 (FIG. 4). P-glycoprotein activity was maintained for more than 100 days in culture with greater than 95% of MDR1_(wt) and MDR1₁₁₉₉ recombinant cell populations excluding R123, as measured by flow cytometry. These cells also retained P-glycoprotein activity after being frozen in liquid nitrogen and thawed.

Characterization of P-glycoprotein Expression in Recombinant LLC-PK1 Cells

After transfection and selection of control and variant 1199 plasmids, it was important to verify MDR1 mRNA and protein expression. The level of MDR1 mRNA transcripts in MDR1 recombinant cells was assessed using an MDR1 standard that was generated with known copy number. MDR1 copy number values were similar in MDR1_(wt) and MDR1₁₁₉₉ recombinant cells with estimations of 2.90×10⁶±0.30 and 3.51×10⁶±0.30 copies of MDR1 mRNA/100 ng total RNA, respectively. Immunoblot analyses of LLC-PK1 cells, normalized by protein concentration, showed approximately equal expression of 150 and 170 kDa bands of P-glycoprotein in MDR1_(wt) and MDR1₁₁₉₉ cells (FIG. 5). Image deconvolution confirmed that MDR1_(wt) and MDR1₁₁₉₉ cells express similar amounts of P-glycoprotein and that expression is predominantly on the apical membrane (FIG. 6).

P-glycoprotein-Mediated R123 Efflux

Efflux transport activity of MDR1_(wt) or MDR1₁₁₉₉ recombinant cells was evaluated by flow cytometry with a fluorescent P-glycoprotein substrate R123. A fluorescent-activated cell sorter (FACS) was used to evaluate P-glycoprotein activity in a population of cells by measuring intracellular R123 accumulation. Mean intracellular R123 fluorescence for MDR1_(wt) and MDR1₁₁₉₉ cells (given in arbitrary units) were 3.91±0.11 and 18.56±0.46 (p<0.001), respectively, an approximate 4.75-fold higher accumulation of R123 in MDR1₁₁₉₉ cells (FIG. 7A). To verify that the R123 efflux observed was due to P-glycoprotein activity, a specific and potent P-glycoprotein inhibitor GF120918 was used for R123 efflux assessment. Although there was differential efflux observed between cells expressing MDR1_(wt) or MDR1₁₁₉₉, R123 efflux activity was completely abolished in both types of cells, reversing cellular R123 accumulation similar to that of control LLC-PK1 cells (FIG. 7B-D). These results demonstrate that recombinant cells expressing MDR1_(wt) or MDR1₁₁₉₉ proteins are both capable of mediating P-glycoprotein-specific R123 transport activity, but cells expressing MDR1₁₁₉₉ are less efficient than those expressing MDR1_(wt).

TABLE 4 Sensitivities of LLC-PK1 control and recombinant MDR1 cells to cytotoxic P-glycoprotein substrates. EC₅₀ values in nM (fold-change from control) LLC-PK1 Cells DOXORUBICIN Vinblastine Vincristine CONTROL 34.2 ± 3.1 0.66 ± 0.22 0.37 ± 0.12 MDR1_(wt)  155 ± 68 (4.53) 1.41 ± 0.51 (2.14) 1.18 ± 0.56 (3.19) MDR1₁₁₉₉  120 ± 32 (3.51) 15.7 + 4.0 (23.8)** 3.41 ± 1.47 (9.23)* *Sensitivity to vincristine of MDR1₁₁₉₉ compared MDR1 wt (p < 0.05). **Sensitivity to vinblastine of MDR1₁₁₉₉ compared MDR1 wt (p < 0.005).

Sensitivity to Cytotoxic Agents

To evaluate the impact of differential efficiency in efflux activity of cells expressing MDR1_(wt) or MDR1₁₁₉₉, three chemotherapeutic agents were employed that have been shown to exhibit P-glycoprotein-dependent drug resistance. Dose response analysis was performed for doxorubicin, vincristine and vinblastine. Drug sensitivity was expressed as the effective concentration necessary to kill 50% of the cells (EC₅₀). EC₅₀ values of the cytotoxic drugs in LLC-PK1 control and MDR1_(wt) and MDR1₁₁₉₉ cells are presented in Table 4. Recombinant cells expressing MDR1_(wt) or MDR1₁₁₉₉, exhibit significantly enhanced resistance to doxorubicin compared to the control cells, but their sensitivity to doxorubicin appears to be similar. A significant degree of resistance toward vinblastine and vincristine was also observed for both MDR1_(wt) and MDR1₁₁₉₉ cells; however, cells expressing MDR1₁₁₉₉ were found to be more resistant to vinblastine and vincristine (about 11- and 2.9-fold respectively; Table 4) than those expressing MDR1_(wt). Collectively, these data indicate that the P-glycoprotein product derived from MDR1₁₁₉₉ exhibits differential sensitivity to vincristine and vinblastine compared to MDR1_(wt) without significantly altering sensitivity to doxorubicin.

Transepithelial Transport

Differences in transcellular directional permeability in epithelial cells expressing MDR1_(wt) and MDR1₁₁₉₉ were evaluated by growing cells on a filter supports, providing for the establishment of polarized monolayers and tight junctions. P-glycoprotein is a directional transporter expressed on the apical membrane of epithelial cells and effluxes substrates in a basolateral-to-apical direction. TEER values, verifying the integrity of the monolayers and formation of tight junctions before an experiment, were 358±47.8 Ω*cm². The directional efflux transport was evaluated with a fluorescent substrate R123.

TABLE 5 Apparent permeability ratios for Rhodamine-123 transport in LLC-PK1 control and recombinant MDR1 cells. P_(appB→A)/P_(appA→B) LLC-PK1 Cells R123 R123 + GF120918 CONTROL 0.903 ± 0.200 1.01 ± 0.33 MDR1_(wt)  3.54 ± 0.94^(†) 0.961 ± 0.216 MDR1₁₁₉₉  2.02 ± 0.51*^(†) 0.561 ± 0.030 *Apparent permeability ratio of MDR1₁₁₉₉ compared to MDR1_(wt) (p < 0.05) ^(†)Apparent permeability ratio of MDR1_(wt) and MDR1₁₁₉₉ compared to LLC-PK1 control (p < 0.01)

The permeability ratios (P_(appB→A)/P_(appA→B)) were calculated for comparison. If cells display directional efflux mediated by P-glycoprotein, a ratio greater than 1.0 is expected, and in the presence of a P-glycoprotein-specific inhibitor, GF120918, the ratio should be reduced to near 1.0. Both MDR1_(wt) and MDR1₁₁₉₉ cells show increased basolateral-to-apical efflux and permeability ratios compared to control cells (FIG. 8 and Table 5). However, cells expressing MDR1₁₁₉₉ appear to have decreased directional efflux and permeability ratios of R123 compared to MDR1_(wt). In the presence of GF120918, a specific P-glycoprotein inhibitor, directional transport is eliminated, suggesting that the directional efflux is P-glycoprotein mediated (FIG. 8 and Table 5).

Discussion

Due to the broad substrate specificity and localization in tissues, the importance of P-glycoprotein-mediated efflux transport in drug delivery and disposition is becoming increasingly appreciated. Significant progress has been made in the discovery of MDR1 polymorphisms and the assessment of allelic frequencies. The search for key genetic determinants, including MDR1 genetic polymorphisms, which alter the disposition of drugs that are substrates or inhibitors of P-glycoprotein in individuals, has just begun. Toward this end, stable recombinant LLC-PK1 cells expressing either MDR1_(wt) or MDR1₁₁₉₉, have been developed for evaluating the significance of the G1199A polymorphism.

Alterations in the efflux transport and chemoresistance of P-glycoprotein due to the G1199A transition have been observed in the recombinant expression systems described herein. Variations in efflux were demonstrated with the fluorescence substrate R123; cells expressing MDR1₁₁₉₉ displayed decreased efflux of R123 compared to MDR1_(wt). Regardless of the efficiency of efflux, cells expressing either MDR1_(wt) or MDR1₁₁₉₉ were both inhibited to the same degree by a P-glycoprotein-specific inhibitor, GF120918. This indicates that the observed differences were due to the influence of an MDR1 polymorphism on P-glycoprotein function. Variation in chemoresistance due to G1199A polymorphism was also observed (Table 4). Cells expressing MDR1₁₁₉₉ appear to be more resistant to vinblastine and vincristine than cells expressing MDR1_(wt); however, MDR1_(wt) and MDR1₁₁₉₉ recombinant cells displayed similar resistance to doxorubicin. Site-directed mutagenesis studies have also shown that modifications in the nucleotide sequence of MDR1 alters substrate efflux of some anticancer agents, but does not cause a change in efflux of others (Loo et al., J. Biol. Chem. 268:19965-72 (1993); Loo et al., J. Biol. Chem. 268:3143-9 (1993); Loo et al., J. Biol. Chem. 269:7243-8 (1994); Loo et al., Biochem. 33:14049-57 (1994)). Multiple binding domains in the drug-binding pocket of P-glycoprotein have been proposed and may account for the drug-specific alteration in P-glycoprotein efflux due to genetic polymorphisms (Martin et al., Mol. Pharmacol. 58:624-32 (2000); Loo et al., J. Biol. Chem. 268:19965-72 (1993); Loo et al., J. Biol. Chem. 268:3143-9 (1993)). The observed differential sensitivity to cytotoxic agents due to G1199A may be important in modulating the efficacy of chemotherapy. The reported results could be used to provide alternate choices of drugs to overcome chemoresistance in some cancer patients. The use of LLC-PK1 epithelial cells as host cells allows for assessment of the role of MDR1 genetic variation in P-glycoprotein-dependent directional transcellular efflux. Permeability and directional efflux transport studies with the epithelial cells demonstrated that MDR1₁₁₉₉ recombinant cells exhibited a lower P_(appB→A)/P_(appA→B) ratio for R123 (Table 5). Changes in drug permeability may impact absorption, bioavailability, and distribution into target tissues, including the CNS.

The variations in P-glycoprotein activity due to G1199A found in the recombinant expression system are not comparable to those reported on MDR1₁₁₉₉ expressed in HeLa cervical adenocancinoma cells using a vaccinia virus-based transient expression system (Kimchi-Sarfaty et al, Mol. Pharmacol. 62:1-6 (2002)). However, the HeLa cell system cannot evaluate transcellular permeability, which is an especially important component of P-glycoprotein activity, particularly in intestinal absorption and blood-brain-barrier penetration. In this transient expression system, HeLa cells expressing MDR1₁₁₉₉ did not exhibit significant differences in efflux of bodipy-FL-verapamil, bodipy-FL-vinblastine, calcein-AM, bodipy-FL-prazosin, bisantrene, and bodipy-FL-forskolin, and daunorubicin (Kimchi-Sarfaty et al., supra.). However, the fluorescent bodipy modification on the P-glycoprotein substrates in this study may influence the ability to detect functional variability due to MDR1 polymorphisms. It is also possible that expression of some vaccinia viral elements and cytolytic viral proteins in these transient expressed HeLa cells contribute to the discordance. The system allows for long-term expression of P-glycoprotein variants without the complication of viral elements.

The role of stable recombinant epithelial cells for expression of P-glycoprotein and variants should not be underestimated. Data in literature contains many contradictory reports of in vitro studies performed to evaluate the influence of MDR1 polymorphisms (primarily G2677T and C3435T) on P-glycoprotein efflux at the cellular level. In vitro retroviral vectors containing MDR1 variants at 2677, designed to express Ala893 or Ser893 in NIH-3T3 mouse embryonic cells, suggested that Ser893-expressing cells exhibited decreased digoxin accumulation than Ala893 cells indicating enhanced P-glycoprotein efflux (Kim et al., Clin. Pharmacol. Ther. 70(2):189-99 (2001)). On the other hand, a vaccinia virus-based transient expression system of several MDR1 polymorphisms (A61G, T307C, G1199A, G2677T, and G2995A) in HeLa cells showed no difference in P-glycoprotein expression and efflux of various drugs (Kimchi-Sarfaty et al., Mol. Pharmacol. 62:1-6 (2002)). Similarly, expression of different combinations of variations at 2677 and 3435 in LLC-PK1 cells (2677G/3435C, 2677G/3435T, 2677A/3435C, 2677A/3435T, 2677T/3435C, 2677T/3435T) were shown to have no apparent difference in transcellular transport and accumulation of verapamil, digoxin, vinblastine, and cyclosporine (Morita et al., Biochem. Pharmacol. 65:1843-52 (2003)). At present, the data in the literature provide no consistent information on the influence of MDR1 polymorphisms in vitro.

There are several reasons that can account for the discrepancies observed between the in vitro observations of various groups of researchers. As described earlier, multiple binding domains may exist in P-glycoprotein, and variations in the amino acid sequence of the protein may alter binding and efflux of some drugs but not others. Therefore, the selection of substrates may be important in assessing changes in P-glycoprotein activity and could explain some observed differences. Also, some vectors carry unique membrane components that may influence the activity of P-glycoprotein. The cells used to express the variant MDR1 genes can also have variable intrinsic transporters, many of which contribute to the efflux of P-glycoprotein substrates. The recombinant expression system we have developed to generate cell lines that continuously express MDR1 variants will be useful for study in detailing the complex interactions of MDR1 polymorphisms.

The expression system described herein, including MDR1 mammalian expression vectors and epithelial host cells capable of reproducibly expressing P-glycoprotein variants and differentiating into polarized monolayers. This strategy can be readily adapted to investigate other SNPs and haplotypes of MDR1 and provide insight into the functional significance on the more than 30 SNPs reported in the literature. For instance, the influence of C3435T can be evaluated to determine whether or not this synonymous polymorphism changes expression or activity of P-glycoprotein, with which it has been reported to be associated. In addition, the effect of MDR1 haplotypes can also be evaluated in this system since multiple SNPs can be expressed at once in the same plasmid. Multiple SNPs found on the same chromosome are assigned to a specific haplotype, and some attempts have been made to determine the role of MDR1 haplotypes in P-glycoprotein variability but the data is still inconclusive (Kim et al., Clin. Pharmacol. Ther. 70(2):189-99 (2001); Johne et al., Clin. Pharmacol. Ther. 72:584-94 (2002); Kroetz et al., Pharmacogenetics 13:481-94 (2003)). The expression systems can be useful in sorting out the role of MDR1 haplotypes.

Mammalian epithelial host cells similar to LLC-PK1, including MDCKII and Caco-2, can be used for evaluating the significance of MDR1 polymorphisms (Polli et al., J. Pharmacol. Exp. Ther. 299:620-8 (2001); Williams et al., J. Pharm. Sci. 92:1957-67 (2003); Zhang et al., Pharm. Res. 15:1520-4 (1998); Stormer et al., Drug. Metab. Dispos. 29:954-6 (2001)). The same MDR1 and variant constructs can be used to generate recombinant expression systems with these cells. The described stable recombinant cells have provided a unique method for evaluating MDR1 genetic polymorphisms. Since LLC-PK1 recombinant cells can be grown continuously in culture for several passages, they can serve as an in vitro tool to detect potential drug-drug interactions in drug development. This continuous system focuses on genetic modifications in the coding region of MDR1 that alter the structure of P-glycoprotein. In addition, other strategies specifically designed to evaluate the effects of promoter and intronic SNPs can be used to address regulation of MDR1 expression.

EXAMPLE 2 Impact of G1199T on Pgp-Mediated Drug Resistance Methods

Cell Culture

Human embryonic kidney cell, HEK293 cells (with or without Pgp expression) were grown at 37° C. in complete media consisting of DMEM medium (Invitrogen, Carlsbad, Calif.) supplemented with 10% (v/v) fetal calf serum and 1% (v/v) antibiotic-antimycotic, 1% non-essential amino acid, and under 5% CO₂.

Generation of MDR1_(wt) and MDR1₁₁₉₉ Plasmids

Total RNA was extracted from the MDR1-overexpressing cell line, MES-SA-DX5 and full-length MDR1 cDNA was generated using the high-fidelity protocol developed previously (Yang et al., Biotech. 33:196, 8, 200 passim. (2002)). The isolated MDR1 cDNA was cloned into a linearized pcDNA-TA vector (Invitrogen, Carlsbad, Calif.) containing cytomegalovirus (CMV) and T7 promoters capable of transcription also described previously (Yang et al., supra.). The plasmid stock was designated as MDR1_(wild-type) (MDR1_(wt)).

The mammalian expression plasmid containing the G1199A or G1199T polymorphism was generated by site-directed mutagenesis (Stratagene, La Jolla, Calif.). Details of this expression plasmid designed to express Pgp in mammalian cells have been published (Woodahl et al., 2004). Briefly, for G1199A, the primers used are: Forward 5′-3′CAGAAATGTTCACTTCAATTACCCATCTCGAAAAG (SEQ ID NO:6), and Reverse 5′-3′ TTTCGAGATGGGTAATTGAAGTGAACATTTCTG (SEQ ID NO:7). For G1199T, the primers are: forward 5′-3′CAGAAATGTTCACTTCATTTACCCATC TCGAAAAG (SEQ ID NO:10), and reverse 5′-3′ TTTCGAGATGGGTAAATGAAGTG AACATTTCTG (SEQ ID NO:11). Clones were screened by restriction enzyme mapping with EcoRI and Bam HI and the sequence was verified with an automated DNA sequencer based on Big-Dye 3.0 chemistry (Applied Biosystems, Foster City, Calif.). The variant expression plasmids were designated as MDR1_(1199A) or MDR1_(1199T).

Isolation of Stable Recombinant Cells Expressing MDR1_(wt) or MDR1₁₁₉₉

The MDR1_(wt), MDR1_(1199A), or MDR1_(1199T) plasmids were introduced into the mammalian cell, HEK293 by electroporation. Ten million cells with 10 μg of plasmid DNA (either MDR1_(wt) or MDR1₁₁₉₉) were suspended in a 0.4 cm electroporation cuvette containing HBSS. Electroporation was performed at 250 V and 975 μF in a Gene Pulser® II (Bio-Rad Laboratories, Hercules, Calif.). Subsequently, cells were kept at 4° C. for 10 min and before returning to the original cell culture conditions. These cells were initially exposed for 3 days with 300 μg/mL G418, a neomycin derivative (Calbiochem, San Diego, Calif.) to select for cells that express neomycin under the direction of the same plasmid vector. The G418 resistance cells were further expanded and verified to have stable, high levels of Pgp expression. These cells, verified to stably express high levels of Pgp are used subsequently for functional studies.

Characterization of P-gp Expression

Absolute quantitation of MDR1 mRNA transcripts in cellular samples was performed using the ABI Prism 7900HT Sequence Detection System (Applied Biosystems). An MDR1RNA standard was generated by the method described previously, and concentration was measured by absorbance at 260 nm and converted to the number of copies of MDR1RNA by the molecular weight (Yang et al., Biotech. 33:196, 8, 200 passim. (2002)). A dilution series of the MDR1RNA standard was used to generate a standard curve of the number of copies of MDR1 mRNA vs. C_(T) value. Following RNA isolation, 100 ng of total RNA of each cellular sample was analyzed in triplicate to obtain a C_(T) value and estimation of the number of copies of MDR1 mRNA.

Immunoblot analysis was also used to verify protein expression. Briefly, 2×10⁶ control and recombinant Pgp expressed cells were pelleted and lysed in a lysis buffer containing SDS and β-mercaptoethanol. Protein concentration was measured by a microplate assay protocol (Bio-Rad Laboratories). Electrophoresis and transfer was run according to instructions for Mini-PROTEAN® II Electrophoresis Cell and Mini Trans-Blot® Electrophoretic Transfer Cell (Bio-Rad Laboratories). Non specific binding sites on the membrane were blocked with 5% evaporated milk in TTBS buffer (0.1% Tween 20; 20 mM Tris-HCl, 0.9% NaCl, pH 7.6). Subsequently the nitrocellulose membrane was incubated with the F4 (Sigma, St. Louis, Mo.), anti-P-gp monoclonal antibody followed by a secondary HRP-conjugated goat anti-mouse IgG. ECL reagents were used as a substrate and blots were exposed to X-ray film to detect anti-Pgp reactive protein bands.

Cell surface expression of P-gp was also analyzed by flow cytometry. Briefly, 5×10⁵ cells were plated overnight on 6-well plates (Corning, Corning, N.Y.). Cells were first washed with phosphate buffered saline with 0.5% bovine serum albumin (PBS/BSA) and detached after 10 min incubation with 2 mM EDTA in PBS. They were incubated with either anti-P-gp monoclonal F4 or a control (matched-isotype mouse) antibody for 30 min at 4° C., followed by three washes with cold Hanks' balanced salt solution supplemented with 0.5% BSA and 0.1% NaN₃ (HBSS-BSA-NaN₃). Cells were then incubated with phycoerythrin (PE)-conjugated goat-anti-mouse IgG (Calbiochem, San Diego, Calif.) for 30 min on ice in the dark, again followed by three washes with HBSS-BSA-NaN₃. Cells were resuspended in HBSS-BSA-NaN₃ and cell surface fluorescence was analyzed with a FACScan flow cytometer (Becton Dickinson, Franklin Lakes, N.J.) using CellQuest software (Becton Dickinson). Pgp expressed cell distribution was determined after accounting for cells that were reactive to non-specific antibody binding.

For immunofluorescence staining and confocal microscopic studies, 4×10⁵ cells were plated on glass chamber slides (8-well; Lab-Tek™; Nalge Nunc International, Rochester, N.Y.) and grown for 4 days. Cells were fixed with 2% paraformaldehyde in PBS (pH 7.4) and permeabilized with 0.2% Triton X-100. Blocking was performed with 2% goat serum in PBS containing 1% BSA (PBS/BSA). Slides were immunostained with anti-P-gp monoclonal antibodies F4 (Sigma) and C219 (Calbiochem) followed by goat anti-mouse secondary antibody, Alexa-594 (Molecular Probes, Eugene, Oreg.). Slides were mounted with Fluoromount-G (Southern Biotechnology, Birmingham, Ala.) and images were collected with a Zeiss Axiovert 200 MAT deconvolution microscope (Carl Zeiss, Oberkochen, Baden-Wuerttemberg, Germany) with a 63× oil immersion objective lens, and analysis was performed with SlideBook 3.0® software (Intelligent Imaging Innovation, Denver, Colo.).

Intracellular Accumulation Assay

One million cells per well were plated overnight on 6-well plates (Corning). For Rhodamine-123 (R123) accumulation, cells were washed with PBS and incubated for 90 min in 5 μM R123 (Sigma) in serum-free RPMI medium 1640 at 37° C. Cells were again washed with PBS and then allowed to efflux for 60 min in complete media. After efflux, cells were washed in ice-cold PBS, trypsinized, and resuspended in 1% paraformaldehyde. Cells were analyzed with a FACScan flow cytometer (Becton Dickinson) using CellQuest software (Becton Dickinson). Ten thousand cells from each sample were analyzed for forward scatter, side scatter, and R123 accumulation.

Inhibition analyses were performed with the specific inhibitor GF120918 [N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl-]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide]. Inhibitory potency was evaluated by calculating concentration necessary for 50% inhibition (IC₅₀) values as the percent inhibition of R123 efflux in the presence of GF120918 over a concentration range from 2 nM to 2 μM. The IC₅₀ values were estimated using a sigmoid Emax model on WinNonlin® software (Pharsight, Mountain View, Calif.). Complete inhibition was performed at 1 μM GF120918.

Doxorubicin accumulation was performed in quadruplicate by incubating cells with 10 μM doxorubicin in PBS with 0, 20, 100, or 1000 nM GF120918 for 1 hour at 37° C. Cells were washed with ice-cold PBS, trypsinized, and lysed in 0.5% deoxycholic acid. To precipitate the proteins tricholoracetic acid was added to 5% and the cell lysates were incubated for 30 min at 4° C. and centrifuged at 20,000 g. The supernatant was removed and fluorescence intensity of doxorubicin was measured with a fluorescence spectrophotometer F-4500 (Hitachi Instruments, San Jose, Calif.); excitation was set at 488 nm and emission at 560 nm.

Cytotoxic Drug Sensitivity Assay

HEK293 control and MDR1 recombinant cells were plated at a density of 1×10⁴ cells/well in a 96-well plate in complete media and allowed to attach for 4 hours at 37° C. Varying concentrations of doxorubicin, paclitaxol (Sigma Aldrich), vinblastine (Bedford Laboratories, Bedford, Ohio), and vincristine (Faulding, Paramus, N.J.) were added to the cells in triplicate. Cells were incubated with the cytotoxic drugs for 3 days at 37° C. On day 3, the media was removed and replaced with OptiMEM. The MTS cell viability assay was performed and cytotoxicity was measured as the effective concentration necessary for 50% cell growth inhibition (EC₅₀) for each drug; EC₅₀ values were estimated using a sigmoid Emax model on WinNonlin® software (Pharsight, Mountain View, Calif.).

Results

Identification and Validation of G199T Variant in Leukemia Patients

As a part of an effort to identify MDR1 coding sequence variations, MDR1RNA from lymphoblast and bone marrow cells of leukemia patients was isolated and the entire 3.8 kb regions of the MDR1RNA were sequenced. These sequence data were confirmed by comparing the DNA sequence data derived from the forward and reverse sequences derived from positive and negative DNA strands. Based on the MDR1 sequence data collected from 44 leukemia patents, two subjects were found to exhibit a G-to-T transition at 1199 nucleotide sequence, which has not been reported. These two individuals are heterozygote for this single nucleotide variation. A higher number of patients who exhibit a G-to-A variation at the same 1199 nucleotide position were also noted. As shown in Table 6, the novel variant 1199T is found with a estimated frequency of 2.3% and the previously reported variant, 1199A is recorded at 9.1% frequency. The frequency of G1199A in these patients is about 2 fold higher than that reported in healthy Caucasians (ref), and 3 fold higher than the newly discovered G1199T variant

Development and Characterization a Recombinant Cell Expression MDR1 and Variants

Expression plasmid carrying MDR1 and its variants were constructed as described in Materials and Methods, and they were expressed in a common mammalian host, Human Embryonic Kidney (HEK) cells. The HEK host cells were chosen based on low or minimal levels of MDR1-like functions and high levels of recombinant protein expression. After selection of recombinant cells with high degree of expression, all the recombinant cells expressing MDR1 and 1199 variants were found able to transport Pgp probe substrate R123. As shown in Table 7, these recombinant cells exhibit varying ability to reduce intracellular accumulation of Pgp probe substrate R123.

These cells were expanded and tested initially for their ability to alter the cytotoxicity profile, presented as 50% inhibitory concentration or EC₅₀ of vinblastine, a Pgp substrate. Stability of the Pgp expression over time was also monitored by retesting the IC₅₀ of different variants over a 2 month period, roughly 15-20 passages. As shown in Table 8, the recombinant cells expressing MDR1 and 1199 variants exhibit higher degree of resistance to vinblastine than the control HEK host cells. In addition, the degree of resistance were sustained over 2 month in culture suggesting the stable expression of the MDR1 gene product, Pgp.

The protein product of MDR1, 1199A and 1199T variants expressed in these stable recombinant HEK cells were further characterized by immunoblot to detect the size of the protein and degree of expression. As shown in FIG. 9, the apparent molecular weight of MDR1_(wt), MDR1_(1199A) and MDR1_(1199T) gene products were similar and recorded to at approximately 170 kD. The recombinant cells were also analyzed by flow cytometery to characterized cell-surface expression of Pgp. As shown in FIG. 10, all the MDR1 expressing HEK recombinant cells exhibit similar degree of surface staining to anti-Pgp antibody suggesting that they express similar levels of surface Pgp, accessible to anti-Pgp antibody.

Collectively, these data suggest that the recombinant HEK cells expressing MDR1 and 1199 variants produced 170 kD product found on cell-surface with similar levels and their expression is stable in culture for at least two months. These cells were used subsequently for determination of variations in drug resistance.

Evaluation of Functional Impact on G1199T Variant on MDR1-Dependent Drug Resistance

Previously with recombinant LLC-PK1 cell expressing MDR1 and variants, we have shown that G1199A coding for an Asn at 400 exhibits a functional variant on the transport of vinblastine and vincristine. Both of these drugs showed a significant impact in resistance as determined by variations in EC₅₀ values and transepithelial transport across monolayer. However, Doxorubicin had only a slightly higher transport rate in the G1199A. To determine whether G1199T exhibits similar or different intracellular efflux transport activity, a panel of drugs was used to generate dose-response profile of recombinant HEK cells expressing MDR1-wild type, G1199A or G1199T variants. A representative dose-response profile for MDR1 substrate doxorubicin and control substrate ivermectin are presented in FIG. 11. As shown in FIG. 11, expression of MDR1 exhibit right-shift or increasing concentration of doxorubicin needed to inhibit cell growth, and G1199A variant further enhance this effect. In contrast, G1199T variant appeared to reduce drug-resistance effect mediated by MDR1 expression. The MDR1 mediated effect was specific in that control drug substrate, ivermectin (which is a BCRP specific substrate) exhibit similar degree of sensitivity among all cells, including non-MDR1 expressing control cells.

Drug sensitivity profile generated for recombinant cells expressing Pgp and variants were summarized in Table 9 for the common chemotherapeutic agents, vinblastine, vincristine, doxorubicin, paclitaxel and ivermectin. With the exception of non-Pgp substrate, ivermectin, all chemotherapeutic agents exhibit lower sensitivity (higher EC50 value) for recombinant cells expressing MDR1. The cell expressing G1199A exhibit significantly higher degree of resistance to all but ivermectin than those expressing wt counter part. In contrast, the G1199T-newly discovered variant exhibit lower sensitivity than that or wt counter part. These results were replicated in at least three separate experiments to be reproducible with less than 10% error between experiments.

Collectively, dose response studies with a panel of chemotherapeutic agents indicate that the newly discovered G1199T variant is less effective in inducing drug resistance phenotype in recombinant cells expressing MDR1 product Pgp. In contrast, the previously reported G1199A variant of MDR1 produces higher resistance than the wt phenotype.

Differences in Intracellular Drug Substrate Concentrations Among MDR1 and 1199 Variants

To determine whether the observed variable increase in drug resistance is due to reduction in intracellular concentrations of Pgp substrates, the time course profile of intracellular concentration of doxorubicin as a model substrate was evaluated. Over a 2 hr time course, a much lower intracellular concentrations of doxorubicin in cells expressing MDR1 was found. While the time to reach maximum intracellular concentrations was similar, the extent of drug accumulation in cells were significantly higher in control cells that lack MDR1, followed by those that express MDR1_(1199T), MDR1_(wt), or MDR1_(1199A). Regardless of which MDR1 variant, reduction in intracellular doxorubicin concentration in recombinant cells expressing MDR1 can be reversed with a well-studied Pgp specific inhibitor, GF120918. Taken together, these data suggest that variation in drug resistance in recombinant cells expressing MDR1 and 1199 variants were likely due to alteration in effectiveness in reduction of intracellular drug concentrations.

Discussion

Capitalizing on the discovery of a genetic variant, G1199T of multidrug resistance protein MDR1 in patients with leukemia, and available stable recombinant cells for systematic evaluation, the functional impact of this single nucleotide polymorphism was characterized. While details mechanisms remained to be explored, probing of dose-response and intracellular concentration of model drug substrates indicates that G1199T variant is less effective in reducing intracellular drug concentrations and therefore, exhibit lower degree of drug resistance, compared to the MDR1_(wt). With MDR1_(G1199T) about 2-3 fold in resistance is found consistently for the for Pgp substrates—doxorubicin, vincristine, vinblastine, and paclitaxel, compared to the MDR1_(wt) (Table 8). In contrast, another variant MDR1_(G1199A) polymorphism, exhibit higher degree of drug resistance than MDR1_(wt) phenotype. Based on these data, it is likely that response rate of patients who are on chemotherapeutic agents that are Pgp substrates, such as paclitaxel, doxorubicin, vincritine, or vinblastine may be significantly different depending on which version of MDR1₁₁₉₉ one expresses.

For the discovery of MDR1 variants, we have focused on analyzing the coding regions in leukemia patents to accelerate the identification of sequence variants that may produce amino acid change and more importantly function of the MDR1 proteins or Pgp. In a relatively small pool of AML subjects, we found the frequency of the G1199T variant to be 2.3%, which is about one-fourth frequency to that of G1199A (9.1%) in the same set of subjects. In our study subjects, we found the frequency of G1199A to be about two fold higher than larger population study in healthy volunteers (˜5.5% in Caucasians). Whether the age of our AML patients may also contribute to a higher frequency of G1199A is not known and age dependent effects remained to be explored. In addition, whether there is any linkage with this G1199T variant with other genetic variation in the same chromosome—either in exon or intron region—exists is not clear, and warrant further evaluation of this gene which has been estimated to be large and characterized recently to be around 200 kb. Also, the frequency of G1199T in healthy human subjects is yet to be determined.

Regardless of the exact frequency of occurrence for this new single nucleotide variant in humans, it is interesting to note that G1199T have significantly altered the function of MDR1 product, Pgp efflux transporter. The G1199T variant, which gives rise to isoleucine instead of serine at 400 amino acid position, exhibited lower efficiency than the wild-type Pgp in efflux transport of a Pgp substrate doxorubicin. The reduced efficiency in efflux function is also reflected in lower degree of resistance to chemotherapeutic agents in recombinant cells expressing G1199T variant, instead of their wild-type counterpart (Table 8). These findings are contrary to the functional impact of another variant sequenced to identical position, G1199A. We found that G1199A variation, which give rise to aspargine, instead of serine at 400 amino acid position, exhibited higher degree of resistance than the parent Pgp protein to chemotherapeutic agents (Table 8). The drug resistance profile for G1199A is consistent with data collected using recombinant epithelial cells expressing MDR1 and variants. (See, e.g., Woodahl et al., J. Pharmacol. Exp. Ther. 310:1199-207, 2004). Nevertheless, it is interesting to note that different single nucleotide substitution at G1199 position of MDR1 produces either enhancement or reduction in Pgp.

While the detailed molecular mechanisms leading to reduction in drug resistance is not clear, the reduction and enhancement in effectiveness to remove Pgp substrate, doxorubicin in cell expressing G1199T and G1199A variant suggest that 400aa position of Pgp is a critical determinant for efficiency of efflux transport activity of Pgp. Based on sequence analysis and protein topology prediction, the 400aa position is located proximity to nucleotide binding region. While it is possible, whether modification of amino acid from serine to either isoleucine (hydrophobic amino acid) or aspargine (basic amino acid) may have an impact on ATP hydrolysis rate and thereby modifying efficiency of transport is not clear, and this issue is under our current investigation. A more detailed molecular analysis will require purified proteins and obtaining crystallized proteins. The stable recombinant cell system over expressing high levels of MDR1 and variants at consistent levels, such as the one described in this report should accelerate this process.

TABLE 6 Allelic frequency of MDR1 1199 polymorphisms in Leukemia patients Sequence/ Amino Acid Estimated allelic variation at 1199nt variation at 400 Frequency in position position leukemia patients Wild type Ser 0.886 G→A Asn 0.091 G→T Ile 0.023

The RNA from 44 leukemia patients were collected and the MDR1RNA transcripts were reverse-transcribed into complementary DNA. These MDR1DNA-products were sequenced and analyzed to assign the allele for each individual subject. These data were validated and presented as overall allelic frequencies.

TABLE 7 Intracellular levels of the Rhodamine 123 in MDR1 expressed and control HEK cells^(a) Intracellular Rhodamine Cell (Unit/mg protein)^(b) % of control^(c) HEK293 4.66 ± 0.14 100 MDR1_(wt) 2.26 ± 0.06 220 MDR1_(G1199A) 1.44 ± 0.12 340 MDR1_(G1199T) 1.82 ± 0.17 260 ^(a)HEK293 and MDR1 transfected cells were seeded into 12 well tissue culture plates at 5 × 105, and incubated at 37 C. for 48 hours. Medium was removed and the cells were washed briefly with 2 ml HBSS. One milliliter of serum-free medium containing 0.5 uM Rhodamine 123 was added into each well for 30 minutes at 37 C. Medium containing Rhodamine was quickly removed, and the plates were washed twice with DPBSG. One milliliter of medium was added to each well and efflux was performed at 37 C. for 30 minute. After extensive washing of the cells with 5 ml DPBSG for 2 times at 4 C., cells were lysed with 1% Triton X-100. The intracellular Rhodamine 123 fluorescence (λ_(ex) = 485 nm; λ_(em) = 530 nm) was determined and expressed as arbitrary units. ^(b)The mean value of Rhodamine fluorescence unit from each well was normalized by total cellular protein contents. ^(c)Value was converted to the HEK293 cells

TABLE 8 Stability/reproducibility of EC50 over time for cell growth inhibition by Vincristine. nM ± s.d. HEK- HEK- Time since first HEK negative HEK-MDR1 MDR1 MDR1 assessment control WT G1199A G1199T 0 month 4.01 ± 1.29 494 ± 115  683 ± 276  246 ± 84 1 months  6.68 ± 0.448 512 ± 85.1 667 ± 87.5 2 months 10.15 ± 1.04  710 ± 33.3 860 ± 51.8

Cells were seeded at 1×10⁴ cells per well on Day −1 in a 96 well plate. Vincristine was added at varying concentrations on Day 0. After 72 hours, the MTS assay was performed on HEK, WT, and G1199A cells. An EC50 value was calculated ±s.d. This was repeated over 1 and 2 months.

TABLE 9 Effects of MDR1 wild-type and G1199 variant expression on sensitivity of HEK cells to chemotherapeutic agents. EC₅₀ (Mean ± S.D. in nM) Drug HEK Control WT G1199A G1199T Vinblastine 1.45 ± 0.23 127 ± 13.0  243 ± 15.1*** 54.7 ± 7.99**  Vincristine 10.2 ± 1.04 710 ± 33.3  860 ± 51.8* 246 ± 84.3** Doxorubicin 45.5 ± 25.1 1107 ± 112   2179 ± 361* 383 ± 39.0** Paclitaxel 2.67 ± 0.13 128 ± 50   ± 42.7 ± 12.7   Ivermectin 4101 ± 210  3809 ± 27.3  4038 ± 77.6 3837 ± 306.7 

The MDR1 wild-type, G1199A or G1199T variants, along with MDR1 negative control HEK host cells were seeded at 1×10⁴ cells per well in a 96 well plate. They were exposed to varying concentration of vincristine, vinblastine, doxorubicin, paclitaxal or ivermectin for 72 hours and cell viability was determined with an MTS assay. The dose-response curve was fitted as described in Materials and Methods to estimate EC₅₀ values. The variations of each fitted value for the quadruplicated samples were expressed as standard deviation (±S.D.). The differences between each treatment group were analyzed with 2-sided t-test compared to WT, and presented as follows: * p<0.05, ** p<0.005, *** p<0.001.

EXAMPLE 3 Impact of MDR1G1199A Polymorphism on Absorption of HIV Protease Inhibitors Across Epithelial Cells

The standard of care for HIV therapy, known as highly active antiretroviral therapy (HAART), utilizes a combination of drugs that attack the virus through different mechanisms of action. HAART combines the use of reverse transcriptase (RT) inhibitors, either nucleoside or nonnucleoside analogues and protease inhibitors, each targeting different mechanisms of the life cycle of HIV. The RT inhibitors, such as AZT, ddC, ddI, d4T, and 3TC, interfere with the reverse transcription of viral RNA. Protease inhibitors, such as amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir, block the HIV protease cleavage of precursor proteins into units necessary for construction of new mature virions. Clinically, oral HAART decreases plasma viral load to undetectable levels over long-term chronic therapy, indicating it is effective in controlling viral replication (see Hammer et al., N. Engl. J. Med. 337:725-733 (1997); Vittinghoff et al., J. Infect. Dis. 179:717-720 (1999); Palella et al., N. Engl. J. Med. 338:853-60 (1998)).

While HAART is effective in containing plasma viral loads, eventual HIV disease progresses to AIDS occurs even in individuals receiving intensive antiretroviral therapy. It is widely believed that viral sanctuary sites, impermeable to drug therapy, exist in the body that ultimately lead to the viral progression. Potential viral sanctuary sites that have been proposed include lymph lodes and lymphoid tissues and the central nervous system (Kinman et al., J. Acquir. Immune Defic. Syndr. 34:387-97 (2003); Kinman et al., AIDS 18:1363-70 (2004); Cavert et al., Science 276:960-4 (1997); Schacker et al., J. Infect. Dis. 181:354-7 (2000)).

P-gp, the gene product of MDR1, has been shown to mediate the transport of HIV protease inhibitors in both in vitro and in vivo models (Kim et al., Clin. Pharmacol. Ther. 70:189-199 (2003); Lee et al., Biochemistry 37(11):3594-601 (1998); Srinivas et al., Antimicrob. Agents Chemother. 42:3157-3162 (1998); Williams et al., J. Pharm. Sci. 92:1957-67 (2003); Washington et al., J. Acquir. Immune Defic. Syndr. Hum. Retrovirol. 19:203-9 (1998)). The oral bioavailability of HIV protease inhibitors in systemic circulation is variable: amprenavir 35-90%, indinavir 70%, nelfinavir 70-80%, ritonavir 60-80%, and saquinavir 4% (van Heeswijk et al., Antivir. Ther. 6(4):201-29 (2001)). In addition to the contribution of cytochrome P450 metabolism, the highly efficient active efflux by P-gp of HIV protease inhibitors in the intestine has been proposed to play a major role in limiting their oral bioavailability.

Furthermore, expression of P-gp may play a role in limiting penetration and accumulation of sufficient concentrations of HIV protease inhibitors into viral sanctuary sites. P-gp expression and activity has also been documented in various subtypes of lymphocytes (Laupeze et al., Hum. Immunol. 62(10):1073-80 (2001); Meaden et al., J. Immunol. Methods 262(1-2):159-65 (2002); Parasrampuria et al., Pharm. Re. 18(1):39-44 (2001)) as well as in endothelial cells at the blood-brain-barrier. The P-gp dependent efflux of HIV protease inhibitors decreases intracellular drug accumulation in lymphocytes (Donahue et al., Clin. Pharmacol. Ther. 73(1):78-86 (2003); Hennessy, Antivir. Ther. 9(1):115-22 (2004); Jones, AIDS 15(11):1353-8 (2001)). P-gp at the blood-brain-barrier limits brain permeability of protease inhibitors (van der Sandt, AIDS 15(4):483-91 (2001)). Therefore, P-gp limits HIV protease inhibitor levels in HIV sanctuary sites and may further reduce the effectiveness of antiretroviral therapy in these tissues.

Researchers have evaluated the effect of P-gp expression on HIV disease progression and protease inhibitor drug levels. In HIV patients, data suggests that expression of functionally defective P-gp increases with disease progression (see Andreana et al., AIDS Res. Hum. Retroviruses 12:1457-1462 (1996). Overexpression of MDR1 in HIV positive was shown to correlate to decreased systemic trough concentrations nelfinavir, indinavir, amprenavir, ritonavir, and saquinavir as well as decreased accumulation in lymphocytes (see Chaillou et al., HIV Clin. Trials 3:493-501 (2002)). Studies have also shown that saquinavir, ritonavir, nelfinavir, indinavir, amprenavir and lopinavir induce the expression of P-gp in human lymphocytes, which could act to further decrease intracellular accumulation of protease inhibitors (see Ford et al., J. Antimicrob. Chemother. 52:354-358 (2003); Dupuis et al., HIV Med. 4:338-345 (2003)).

Genetic polymorphisms are now known to play a role in P-gp expression and function. In addition, MDR1 polymorphisms, particularly the C→T transition at nucleotide 3435, have been correlated to HIV protease inhibitor levels and response to antiretroviral drug therapy. Homozygous 3435T patients receiving nelfinavir therapy had lower plasma nelfinavir concentrations homozygous 3435C patients Fellay et al., Lancet 359:30-6 (2002)). In addition, 3435T patients had a significantly greater rise in CD4+ cell counts and a trend towards lower viral loads than 3435C patients (Fellay et al., supra). Other researchers have also found a strong trend associated with homozygous 3435T patients more effectively responding to protease inhibitor therapy (see Brumme et al., AIDS 17:201-208 (2003); Nasi et al., AIDS 17:1696-1698 (2003)). However, it is difficult to determine whether the C3435T polymorphism acts directly at the cellular and molecular level because this polymorphism does not code for an amino acid modification. Therefore, it is important to have a tool with which to probe the penetration and uptake of protease inhibitors at the cellular level. To accomplish this, the influence of MDR1 polymorphisms that lead to an amino acid modification must be evaluated by functional studies at the cellular and molecular levels. Toward this end, a recombinant cell expression system expressing MDR1 G1199A has been developed and G1199A expression has been demonstrated to alter anticancer drug resistance and permeability (Woodahl et al., J. Pharmacol. Exp. Ther. 310(3):1199-207 (2004)).

Using these recombinant cells, the next objective in elucidating the role of G1199A was to evaluate whether G1199A will also have an influence on the transport and permeability of HIV protease inhibitors. Altered permeability of HIV protease inhibitors across an epithelial barrier due to MDR1 polymorphisms may affect absorption of protease inhibitors from the intestine as well as their penetration into potential HIV sanctuary sites such as lymphoid tissues and the central nervous system (CNS).

Methods

Cell Culture

LLC-PK1 control and transfected cells were grown in complete media consisting of RPMI medium 1640 (Invitrogen, Carlsbad, Calif.) supplemented with 10% (v/v) fetal calf serum and 1% (v/v) antibiotic-antimycotic and grown at 37° C. in the presence of 5% CO₂. For deconvolution immunofluorescent microscopy and transepithelial transport studies, cells were grown in Medium 199 (Invitrogen) supplemented with L-glutamine, 10% (v/v) fetal calf serum, and 1% (v/v) antibiotic-antimycotic and grown at 37° C. in the presence of 5% CO₂.

Transepithelia Transport Assay

LLC-PK1 control and recombinant MDR1 cells were plated at a density of 2×10⁶ cells/24 mm well on permeable supports (Transwell™; 3.0 μm membrane pore size; Corning) and grown for 4 days; media was refreshed after two days in culture. Fresh media was added to the cells one hour before the initiation of the experiment, and transepithelial electrical resistance (TEER) values were measured with a Millicell®-ERS (Millipore, Billerica, Mass.). Transepithelial efflux with radiolabeled compounds ³H-amprenavir, ³H-lopinavir, ³H-ritonavir, and ³H-saquinavir (Moravek) were performed at concentrations of 5 μM (1 μCi per well) in Opti-MEM medium (Invitrogen). Efflux of indinavir was also performed at 5 μM and aliquots were diluted 1:1 in methanol for analysis with liquid-chromatography coupled mass-spectroscopy (LC/MS) using an Agilent 1100 LC-MSD fitted with an RX-C8 column (2.1 mm×15 cm; Zorbax, Agilent Technologies, Palo Alto, Calif.). A 10 μL sample was injected onto the column and eluted isocratically at a flow rate of 0.25 mL/min with a mobile phase containing acetonitrile/20 mM acetic acid (65:35, vol/vol). The mass spectroscopy was operated in atmospheric pressure ionization-electrospray ionization (API-ESI⁺) mode, and the analytes were detected using selected-ion monitoring at m/z 614.7-615.7 days to detect indinavir. Inhibition of transport was performed at 1 μM GF120918. Aliquots of 50 μL were taken from apical and basal compartments at 1, 2, 3, and 4 hours for each drug. Percent transport in both directions, apical-to-basolateral and basolateral-to-apical, was calculated at each time point as the amount of drug in the recipient compartment divided by the initial amount in the donor compartment. Apparent permeability (P_(app)) was calculated in the apical-to-basolateral direction (P_(appA→B)) and in the basolateral-to-apical direction (P_(appB→A)) as described (Polli et al., J. Pharmacol. Exp. Ther. 299:620-8 (2001)). Briefly, P_(app)=(1/(A*C₀))*(dQ/dt), where A is the surface area of permeable support, C₀ is the initial concentration in the donor compartment, and dQ/dt is the rate of transfer of compound into the acceptor compartment. The ratio of P_(appB→A)/P_(appA→B) was also estimated to evaluate P-gp-mediated directional efflux.

Results

Directional Transepithelia Transport

Evaluating transport of compounds across an epithelial cell barrier provides the ability to predict how drug will move across physiologic cell barriers in the body, including the intestine, kidney, and blood-brain-barrier. For a physiologic comparison, in intestinal epithelial cells, the apical compartment represents the intestine lumen; in kidney epithelial cells, the apical compartment represents the kidney proximal tubules; and in brain endothelial cells, the apical compartment represents blood adjacent to the blood-brain-barrier. In contrast, in intestinal and kidney epithelial cells the basolateral compartments represents the blood and in the brain endothelial cells the basolateral compartment represents the brain. Drugs administered orally will encounter the apical membrane of intestinal epithelial cells and will have to cross this cellular barrier before entering systemic circulation. Intravenously administered drugs will encounter the basolateral membrane leading to their excretion and elimination.

The transfer of amprenavir, indinavir, lopinavir, ritonavir, and saquinavir over time across an epithelial cell barrier was measured with the drugs alone (apical-to-basolateral: FIG. 12; basolateral-to-apical: FIG. 13) and with the specific inhibitor GF120918 (apical-to-basolateral: FIG. 14; basolateral-to-apical FIG. 15). Drug was placed in either the apical or basolateral compartment and percent transport (amount of drug in recipient compartment/initial amount in donor compartment) was estimated for each time point.

Apical-to-basolateral efflux, in the absence of GF120918, was greatly reduced by the expression of either MDR1_(wt) or MDR1₁₁₉₉ compared to control cells, indicating expression of functional P-gp (FIG. 12). In control cells, movement of drug across a cell membrane is assumed to be mediated primarily by diffusion; however, all protease inhibitors did not cross the epithelial membrane at the same rate in the apical-to-basolateral direction indicating rates of transfer may be a function of physiochemical properties of protease inhibitors. The rank order of transfer (dQ/dt, presented in Table 10) in control cells was amprenavir=lopinavir>indinavir=ritonavir>saquinavir (a significant difference of dQ/dt between protease inhibitors was defined as p<0.05). The rank order of dQ/dt of protease inhibitors was similar between MDR1_(wt) (amprenavir>lopinavir=ritonavir>indinavir>saquinavir; p<0.05) and MDR1₁₁₉₉ cells (amprenavir>lopinavir>ritonavir>indinavir>saquinavir; p<0.05); and corresponded to the control cells.

Next, basolateral-to-apical efflux was evaluated in the three cell types (FIG. 13). Expression of either MDR1_(wt) or MDR1₁₁₉₉ enhanced basolateral-to-apical flux compared to control cells, confirming expression of P-gp. Again, diffusion in the control cells was not equal across the protease inhibitors with the rank order as lopinavir=amprenavir=ritonavir>saquinavir>indinavir (p<0.05). The rank order of dQ/dt in MDR1_(wt) (lopinavir>ritonavir=amprenavir=saquinavir=indinavir; p<0.05) and MDR1₁₁₉₉ (lopinavir>ritonavir=amprenavir>saquinavir=indinavir; p<0.05) also corresponded to the control cells

Differences in the activity of P-gp between cells expressing MDR1_(wt) or MDR1₁₁₉₉ were observed for all five protease inhibitors. The transepithelial data was evaluated in the individual directions (Table 10). A highly significant decrease in apical-to-basolateral flux in MDR1₁₁₉₉ cells was observed for all protease inhibitors compared to MDR1_(wt) cells [amprenavir: 1.7-fold (p<0.0001); indinavir: 1.9-fold (p<0.001); lopinavir: 1.6-fold (p<0.0001); ritonavir: 2.3-fold (p<0.0001); and saquinavir 2.3-fold (p<0.0001)]. These data indicate that P-gp expressed in MDR1_(wt) cells was less efficient at limiting absorption across the epithelial barrier compared to MDR1₁₁₉₉ cells. However, no differences were observed between MDR1_(wt) and MDR1₁₁₉₉ cells in the basolateral-to-apical direction, with the exception of a modest increase in efflux of ritonavir (1.2-fold; p<0.05) in MDR1₁₁₉₉ cells compared to MDR1_(wt). Therefore, while a significant amount of drug was secreted across the epithelial cells in the basolateral-to-apical direction in both MDR1 cells, there appears to be no G1199A genotypic differences. And such a significant amount of drug crossed the cell barrier, especially apparent in the case of lopinavir, that the rate of transfer appeared to diminish over time in both cells (FIG. 13); therefore, the system may not have been under sink conditions at the later time points. Therefore, the data suggests that while both MDR1_(wt) and MDR1₁₁₉₉ recombinant cells display efficient apical-to-basolateral and basolateral-to-apical efflux of protease inhibitors, the influence of the G1199A polymorphism may have the largest impact on the absorptive apical-to-basolateral direction.

The specific P-gp inhibitor, GF120918, was used in the apical-to-basolateral (FIG. 14) and basolateral-to-apical (FIG. 15) directions to confirm the efflux of protease inhibitors was mediated by P-gp. GF120918 had little effect on the control cells in either direction. Inhibition with GF120918 effectively blocked P-gp activity in both MDR1_(wt) and MDR1₁₁₉₉ cells; therefore, enhancing apical-to-basolateral efflux and decreasing basolateral-to-apical efflux for all protease inhibitors. This confirms that the efflux of protease inhibitors in these cells is a P-gp specific effect. In the presence of GF120918, G1199A genotypic differences in apical-to-basolateral efflux were eliminated for all protease inhibitors except for indinavir; and a genotypic difference was also observed in the basolateral-to-apical direction for indinavir. Therefore, other transporters besides P-gp may play a role in the efflux of indinavir across an epithelial membrane.

Collectively, these data indicate that P-gp is the primary contributor of protease inhibitor efflux across the recombinant cells system that has been developed. Significant differences in P-gp efflux between MDR1_(wt) and MDR1₁₁₉₉ cells were observed for all protease inhibitors in the apical-to-basolateral direction but negligible genotypic differences exist in efflux in the basolateral-to-apical direction. G1199A expression had the largest impact on the apical-to-basolateral efflux of ritonavir and saquinavir, followed by indinavir, then amprenavir, and then lopinavir. This highly significant decrease in apical-to-basolateral efflux in MDR1₁₁₉₉ cells was abolished in the presence of GF120918 indicating that the observed G1199A effect was mediated by P-gp.

Apparent Permeability Ratios Across Epithelial Cells

After evaluating P-gp efflux in the individual directions, the next step was to estimate permeability ratios (P_(appB→A)/P_(appA→B)) that represent the net flow of drug across an epithelial membrane. The ratios were calculated for comparison between MDR1_(wt) and MDR1₁₁₉₉ cells (Table 11). If cells display directional efflux mediated by P-gp, a permeability ratio greater than 1.0 is expected, and in the presence of a P-gp-specific inhibitor, GF120918, the ratio should be reduced to about unity. As shown in Table 11, both MDR1_(wt) and MDR1₁₁₉₉ cells exhibit increased permeability ratios compared to control cells for all protease inhibitors, indicating that P-gp enhanced drug efflux. Cells expressing MDR1₁₁₉₉ have significantly increased permeability ratios of all protease inhibitors compared MDR1_(wt) [amprenavir: 1.7-fold (p<0.0005); indinavir: 1.8-fold (p<0.05); lopinavir: 1.5-fold (p<0.0005); ritonavir: 2.8-fold (p<0.0005); and saquinavir 2.1-fold (p<0.0005)]. The rank order of permeability was saquinavir>indinavir>lopinavir>ritonavir>amprenavir in MDR1_(wt) cells. The rank order in MDR1₁₁₉₉ cells was saquinavir>indinavir>ritonavir>lopinavir>amprenavir. The permeability ratios of saquinavir in both MDR1 cells types were substantially higher than the ratios for other protease inhibitors, indicating that saquinavir is very efficiently effluxed by P-gp. In the presence of GF120918, permeability ratios were dramatically reduced in both MDR1_(wt) and MDR1₁₁₉₉ cells, confirmed that efflux was mediated by P-gp. While the permeability ratios are not reduced down to that of the control cells in the presence of GF120918, the numbers are similar between MDR1_(wt) and MDR1₁₁₉₉.

Discussion

The recombinant cell expression system is a highly reliable and consistent method with which to evaluate functional changes in P-gp due to MDR1 genetic variation. Using this system, expression of G1199A was shown to result in variable cellular sensitivity and permeability of anti-cancer agents. This system has been extended to study the influence of G1199A in transepithelial permeability of HIV protease inhibitors. The ability of HIV protease inhibitors to cross a cellular barrier is important not only in appreciating their absorption from the intestine but also their penetration into potential HIV sanctuary sites such as lymphoid tissues and the CNS.

The apical-to-basolateral data indicate that MDR1 expression, and therefore P-gp efflux, dramatically reduces absorption of protease inhibitors from the apical to the basolateral compartment, analogous to the effect P-gp has in limiting absorption of protease inhibitors from the intestine after oral administration. The same effect would also be observed at the blood-brain-barrier, limiting brain penetration, where drug is presented to the apical membrane of polarized cells. The basolateral-to-apical data, which represents the contribution of P-gp to systemic clearance where drug is presented to the basolateral cell membrane (i.e. after absorption of orally administered drug or after intravenous administration), shows that MDR1 expression enhances efflux of protease inhibitors to the apical compartment for excretion and elimination.

The rank orders of protease inhibitor transfer in the apical-to-basolateral and basolateral-to-apical directions did not correspond; however the ranks in each direction were similar between the three cell types. This indicates that different membrane components may exist in the apical or basolateral membranes of the polarized cells that aid in passive diffusion. The observed rank orders in the recombinant cell system correspond to previously reported studies. Apparent permeability ratios estimated in MDR1-MDCKII cells was the largest for saquinavir followed by ritonavir and then amprenavir, consistent with the permeability ratios in this study (Polli et al., J. Pharmacol. Exp. Ther. 299:620-8 (2001)). Directional transport in MDR1-MDCKII and Caco-2 cells also found that saquinavir had lower apical-to-basolateral transport than ritonavir and that the apparent permeability of saquinavir was greater than ritonavir (Troutman, Pharm. Res. 20:1210-24 (2003)). Permeability of saquinavir and indinavir in Caco-2 cells observed that apical-to-basolateral permeability was less for saquinavir than indinavir and that the apparent permeability ratio was greater for saquinavir than indinavir (Balimane, Eur. J. Pharm. Biopharm. 58:99-105 (2004)). Therefore, the LLC-PK1 recombinant expression cell system appears to display similar transport characteristics of HIV protease inhibitors as other in vitro cell systems.

Obvious differences in P-gp mediated efflux of protease inhibitors were observed between MDR1_(wt) and MDR1₁₁₉₉ cells. Bi-directional permeability ratios were statistically significant for all five protease inhibitors, demonstrating that MDR1₁₁₉₉-expressing cells were more efficient at limiting the ability of amprenavir, indinavir, lopinavir, ritonavir, and saquinavir to cross an epithelial membrane. However, this type of net flow analysis with bi-directional permeability ratios makes it difficult to distinguish the exact mechanism by which the G1199A polymorphism alters P-gp activity. Therefore, to further characterize the influence of G1199A on P-gp activity, the effect of G1199A was evaluated independently on either absorptive processes (apical-to-basolateral) or secretory processes (basolateral-to-apical). According to analysis of the rates of transfer of protease inhibitors, the observed changes in permeability ratios were due primarily to changes in apical-to-basolateral efflux between MDR1_(wt) and MDR1₁₁₉₉ cells. All drugs displayed decreased apical-to-basolateral efflux in MDR1₁₁₉₉ cells compared to MDR1_(wt) cells, with the greatest effect observed in efflux of ritonavir and saquinavir, followed by indinavir, amprenavir, and lopinavir. The basolateral-to-apical efflux of protease inhibitors was similar between MDR1_(wt) and MDR1₁₁₉₉ cells, with the exception of a modest increase in basolateral-to-apical efflux of ritonavir in MDR1₁₁₉₉ cells. Therefore, the effect of the G1199A polymorphism appears to be most significant when drug is exposed to the apical membrane of epithelial cells (i.e., absorption and penetration of the blood-brain-barrier) than when it is exposed to the basolateral membrane (i.e., secretion and elimination by kidney and liver).

The P-gp mediated efflux of protease inhibitors observed in both directions in MDR1_(wt) and MDR1₁₁₉₉ cells was almost completely inhibited in the presence of GF120918 near that of the efflux of control cells. As mentioned earlier, complete inhibition was not observed in the case of indinavir, which suggests that other transporters may be involved in the transepithelial efflux of indinavir. These data indicate that the differential transepithelial efflux of protease inhibitors due to G1199A expression observed in the recombinant cell system was mediated by P-gp.

In this chapter, the stable recombinant expression system has been used to further characterize the influence of G1199A on P-gp activity. The mechanisms by which G1199A alters P-gp activity are not yet known. The G1199A polymorphism causes a serine to asparagine change at amino acid 400, which lies adjacent to the first ATP-binding domain of P-gp and to transmembrane domain 6, one of the transmembrane domains important in substrate binding (REF). However, since a crystal structure is not yet available for P-gp, it is difficult to speculate on the three-dimensional structure of P-gp and where amino acid 400 lies in relation to the substrate-binding regions or ATP-binding domain of P-gp. Therefore, the G1199A polymorphism may affect ATPase activity of P-gp, altering ATP hydrolysis necessary for substrate efflux. Since the impact of G1199A was not equal across the protease inhibitors evaluated, expression of G1199A may also directly affect P-gp drug binding, affinity, or transport. Multiple binding sites of P-gp have been proposed so it is probable that amino acid modification, due to MDR1 polymorphisms, may differentially alter efflux of P-gp substrates (REF).

In summary, the transepithelial efflux and permeability of amprenavir, indinavir, lopinavir, ritonavir, and saquinavir has been evaluated in the stable recombinant cell system, and these drugs have been shown to be excellent substrates of P-gp. Expression of MDR1 in LLC-PK1 cells enhances both apical-to-basolateral and basolateral-to-apical efflux of protease inhibitors, and this effect can be reversed with the specific P-gp inhibitor GF120918. A significant impact of the G1199A polymorphism of efflux and permeability, although the magnitude of a G1199A effect is drug specific and the influence appears to have a more dramatic effect in the apical-to-basolateral direction, was also observed. Therefore, the G1199A polymorphism may impact intestinal absorption of protease inhibitors and penetration into cells and tissues of the lymphoid and central nervous systems.

TABLE 10 Rates of Transfer from Donor to Recipient Compartments (dQ/dt ± s.d.) for HIV Protease Inhibitors in LLC-PK1 Control and Recombinant MDR1 Cells Protease Inhibitor Alone Protease Inhibitor + GF120918 A→B B→A A→B B→A Control amprenavir 9.33 ± 0.46 11.6 ± 0.8 10.8 ± 0.9 10.2 ± 0.4 indinavir 7.17 ± 0.03 6.69 ± 0.21 4.26 ± 0.25 5.99 ± 0.11 lopinavir 9.21 ± 0.24 12.1 ± 0.7 8.48 ± 0.16 10.2 ± 0.4 ritonavir 6.46 ± 0.94 11.0 ± 0.5 6.46 ± 0.69 9.60 ± 0.75 saquinavir 3.02 ± 0.35 8.03 ± 0.51 3.48 ± 0.57 7.36 ± 0.60 MDR1_(wt) amprenavir 2.39 ± 0.14 20.3 ± 1.0 9.90 ± 0.74 10.3 ± 0.5 indinavir 0.515 ± 0.038 17.0 ± 0.8 2.33 ± 0.09 6.91 ± 0.23 lopinavir 0.968 ± 0.057 24.9 ± 1.1 7.71 ± 0.35 11.1 ± 0.5 ritonavir 1.04 ± 0.05 17.2 ± 0.7 3.84 ± 0.28 13.6 ± 0.4 saquinavir 0.124 ± 0.013 18.5 ± 1.3 2.03 ± 0.25 13.0 ± 1.3 MDR1₁₁₉₉ amprenavir   1.39 ± 0.11*** 19.4 ± 0.7 8.52 ± 1.26 9.35 ± 0.68 indinavir  0.279 ± 0.068** 15.5 ± 0.8 1.65 ± 0.25* 4.28 ± 0.04*** lopinavir   0.617 ± 0.020*** 23.8 ± 0.8 6.89 ± 0.69 10.3 ± 0.5 ritonavir   0.451 ± 0.016*** 20.5 ± 2.1* 3.68 ± 0.49 12.9 ± 0.7 saquinavir   0.054 ± 0.003*** 16.5 ± 1.6 1.88 ± 0.26 10.7 ± 1.1 ^(a)Significant difference between MDR1₁₁₉₉ and MDR1_(wt) cells; *p < 0.05, **p < 0.001, and ***p < 0.0001.

TABLE 11 Apparent Permeability Ratios for HIV Protease Inhibitors in LLC-PK1 Control and Recombinant MDR1 Cells P_(APPB→A)/P_(APPA→B) LLC-PK1 CELLS CONTROL MDR1_(WT) MDR1₁₁₉₉ AMPRENAVIR 1.24 ± 0.10 8.48 ± 0.65 14.0 ± 1.2*** AMPRENAVIR + 0.94 ± 0.08 1.04 ± 0.09 1.10 ± 0.18 INDINAVIR 0.93 ± 0.03 31.6 ± 2.6 55.4 ± 13.8* INDINAVIR + GF120918 1.41 ± 0.09 2.96 ± 0.15 2.59 ± 0.39 LOPINAVIR 1.31 ± 0.08 25.7 ± 1.9 38.6 ± 1.9*** LOPINAVIR + GF120918 1.21 ± 0.05 1.44 ± 0.09 1.49 ± 0.16 RITONAVIR 1.70 ± 0.26 16.6 ± 1.1 45.4 ± 4.9*** RITONAVIR + GF120918 1.49 ± 0.20 3.54 ± 0.28 3.50 ± 0.50 SAQUINAVIR 2.66 ± 0.35  149 ± 19  306 ± 35*** SAQUINAVIR + 2.11 ± 0.39 6.40 ± 1.03 5.72 ± 0.86 ^(a)Significant difference between MDR1₁₁₉₉ and MDR1_(wt) cells; *p < 0.05, **p < 0.005, and ***p < 0.0005.

TABLE 12 Summary of effect of MDR1-G-1199-A variation on intracellular uptake of select HIV protease inhibitors-Saquinavir, Ritonavir and Amprenavir: Study in recombinant LLC-PK1 cells expressing MDR1 and variants Vmax Km Vmax/ HIV-PI LLC-PK1 (pmole/ min * 10⁶ cells) (μM) Km Ritonavir Control 0.35 0.25 1.40 MDR1-wt 0.29 1.00 0.29 MDR1-1199A 0.35 2.60 0.13 Saquinavir Control 1.47 2.00 0.74 MDR1-wt 1.13 18.00 0.06 MDR1-1199A 1.00 14.5 0.07 Amprenavir Control 0.048 0.25 0.192 MDR1-wt 0.036 10.25 0.004 MDR1-1199A 0.011 1.00 0.011

The previous examples are provided to illustrate but not to limit the scope of the claimed inventions. Other variants of the inventions will be readily apparent to those of ordinary skill in the art and encompassed by the appended claims. All publications, patents, patent applications and other references cited herein are hereby incorporated by reference. 

1. An isolated nucleic acid molecule comprising a nucleic acid sequence comprising at least 15 contiguous nucleotides of a variant of the nucleotide sequence set forth in SEQ ID NO:5, wherein: (i) the nucleic acid sequence comprises the nucleotide at position 163 of SEQ ID NO:5 and said variant encodes T55P; (ii) the nucleic acid sequence comprises the nucleotide at position 886 of SEQ ID NO:5 and said variant encodes N296H; (iii) the nucleic acid sequence comprises the nucleotide at position 1199 of SEQ ID NO:5 and said variant encodes S400I; (iv) the nucleic acid sequence comprises at least one of the nucleotide at position 1292 or position 1293 of SEQ ID NO:5 and said variant encodes C431L; (v) the nucleic acid sequence comprises the nucleotide at position 2814 of SEQ ID NO:5 and said variant encodes F938L; (vi) the nucleic acid sequence comprises the nucleotide at position 3258 of SEQ ID NO:5 and said variant encodes F3258F; or (vii) the nucleic acid sequence comprises the nucleotide at position 3271 of SEQ ID NO:5 and said variant encodes A1091P.
 2. The isolated nucleic acid molecule of claim 1, further comprising a complementary second nucleic acid molecule.
 3. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 163 of SEQ ID NO:5 and said variant comprises A163C.
 4. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 886 of SEQ ID NO:5 and said variant comprises A886C.
 5. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 1199 of SEQ ID NO:5 and said variant comprises G1199T.
 6. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 1292 of SEQ ID NO:5 and said variant comprises G1292T.
 7. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 1293 of SEQ ID NO:5 and said variant comprises T1293G.
 8. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 2814 of SEQ ID NO:5 and said variant comprises T2814G.
 9. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 3258 of SEQ ID NO:5 and said variant comprises C3258T.
 10. The isolated nucleic acid molecule of claim 1, wherein the nucleic acid sequence comprises the nucleotide at position 3271 of SEQ ID NO:5 and said variant comprises G3271C.
 11. The nucleic acid molecule of claim 1, wherein the encoded variant MDR1 protein or a fragment thereof comprises MDR1 activity.
 12. An expression vector comprising the nucleic acid sequence of claim
 1. 13. An isolated cell line expressing an MDR1 protein which is a variant of the wild-type MDR1 protein having the amino acid sequence set forth in SEQ ID NO:4, said MDR1 variant having at least one of the amino acid substitutions selected from the group consisting of T55P, N296H, S400I, C431L, F938L, and A1091P.
 14. The isolated cell line of claim 13, which is a stable recombinant cell line comprising the expression vector of claim
 12. 15. The isolated cell line of claim 13, which is an epithelial cell line.
 16. The isolated cell line of claim 13, which expresses an MDR1 variant having the S4001 amino acid substitution.
 17. A method for evaluating the influence of a MDR1 polymorphism on MDR1 mediated efflux, uptake, or transcellular transport, comprising: (a) contacting a first cell with a therapeutic agent, said first cell expressing an MDR1 variant having at least one of the amino acid substitutions selected from the group consisting of T55P, N296H, S400I, C431L, F938L, and A1091P; (b) contacting a control cell with the therapeutic agent, wherein said control cell expresses the wild type MDR1 protein; (c) culturing the first cell and control cell; (d) determining for each of the first cell and control cell at least one of the uptake, efflux, or transcellular transport of the therapeutic agent; and (e) comparing the level of uptake, efflux, or transcellular transport of the therapeutic agent in the first cell with that of the control cell.
 18. A stably transformed cell line capable of transcellular transport and comprising a non-viral vector encoding and expressing a recombinant mammalian MRP1 or MDR1 protein, or a homolog thereof.
 19. The cell line of claim 18, wherein the cell line is a renal or intestinal epithelial cell line.
 20. The cell line of claim 18, wherein the epithelial cell line is LLC-PK1, MDCKII, or Caco-2.
 21. A method for evaluating the contribution of an MRP1 or MDR1 protein to the disposition or penetration of an agent, comprising: (a) contacting a stably transformed cell line capable of transcellular transport and comprising an expression vector encoding and expressing a recombinant mammalian MRP1 or MDR1 protein, or a homolog thereof, with the agent; (b) culturing the contacted cell line; and (c) determining at least one of the uptake, efflux, or transcellular transport of the agent.
 22. A method for evaluating the influence of a MDR1 or MRP1 polymorphism on MDR1- or MRP1-mediated efflux, uptake, or transcellular transport, comprising: (a) contacting a first stably transformed cell capable of transcellular transport and comprising an expression vector encoding and expressing a recombinant mammalian allelic variant of a wild-type MRP1 or MDR1 protein, or a homolog thereof, with a therapeutic agent; (b) contacting a control cell with the therapeutic agent, wherein said control cell expresses the wild type MRP1 or MDR1 protein corresponding to the allelic variant; (c) culturing the first cell and control cell; (d) determining for each of the first cell and control cell at least one of the uptake, efflux, or transcellular transport of the therapeutic agent; and (e) comparing the level of uptake, efflux, and/or transcellular transport of the therapeutic agent in the first cell with that of the control cell.
 23. The method of claim 22, wherein the stably transformed cell comprises an expression vector encoding and expressing a recombinant mammalian allelic variant of a wild-type MDR1 protein, or a homolog thereof.
 24. The method of claim 22, wherein the stably transformed cell comprises an expression vector encoding and expressing a recombinant mammalian allelic variant of the wild-type MRP1 protein, or a homolog thereof. 